Update PULL_REQUEST_TEMPLATE.md

Some improvements to the pull request template:
- require sentence case for short description, for consistency
- suggest submitting one item per pull request, to ease review
- improve template formatting, for pull request readability

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,23 +2,30 @@ Thank you for taking the time to work on a PR for Awesome-Selfhosted!
 
 To ensure your PR is dealt with swiftly please check the following:
 
-- [ ] Your submissions are formatted according to the following requirements (`Demo` and `Clients` are optional, don't add a duplicate `Source code` link if it is the same as the main link):
-
-``- [Name](http://homepage/) - Short description, less than 250 characters. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `License` `Language` ``
-
-- [ ] If your additions depend on proprietary services outside the user's control, they must be marked `⚠`.
-
-``- [Name](http://homepage/) `⚠` - Short description, less than 250 characters. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `License` `Language` ``
-
-- [ ] Additions that are not [Free software](https://en.wikipedia.org/wiki/Free_software) must be added to `non-free.md` and marked `⊘ Proprietary`:
-
-``- [Name](http://homepage/) `⊘ Proprietary` - Short description, less than 250 characters. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `Language` ``
-
-- [ ] Your additions are ordered alphabetically.
-- [ ] Your additions are not already listed at [awesome-sysadmin](https://github.com/n1trux/awesome-sysadmin) (IT infrastructure management), [awesome-analytics](https://github.com/onurakpolat/awesome-analytics) (analytics), [staticgen.com](https://www.staticgen.com/) or [staticsitegenerators.net](https://staticsitegenerators.net/) (static site generators).
-- [ ] The `Language` tag is the main server-side requirement for the software - don't include frameworks/specifc dialects.
-- [ ] Any licenses you have added are in our [list of licenses](https://github.com/awesome-selfhosted/awesome-selfhosted/blob/master/README.md#list-of-licenses).
+- [ ] Submit one item per pull request. This eases reviewing and speeds up inclusion.
+- [ ] Format your submission as follows, where `Demo` and `Clients` are optional.
+  Do not add a duplicate `Source code` link if it is the same as the main link.
+  Keep the short description under 250 characters and use [sentence case](https://en.wikipedia.org/wiki/Letter_case#Sentence_case)
+  for it, even if the project's webpage or readme uses another capitalisation
+  such as title case, all caps, small caps or all lowercase.
+  ``- [Name](http://homepage/) - Short description, under 250 characters, sentence case. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `License` `Language` ``
+- [ ] Additions that depend on proprietary services outside the user's control must be marked `⚠`.
+  ``- [Name](http://homepage/) `⚠` - Short description, under 250 characters, sentence case. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `License` `Language` ``
+- [ ] Additions that are not [Free software](https://en.wikipedia.org/wiki/Free_software)
+  must be added to `non-free.md` and marked `⊘ Proprietary`:
+  ``- [Name](http://homepage/) `⊘ Proprietary` - Short description, under 250 characters, sentence case. ([Demo](http://url.to/demo), [Source Code](http://url.of/source/code), [Clients](https://url.to/list/of/related/clients-or-apps)) `Language` ``
+- [ ] Additions are inserted preserving alphabetical order.
+- [ ] Additions are not already listed at any of
+  - [awesome-sysadmin](https://github.com/n1trux/awesome-sysadmin) (IT infrastructure management),
+  - [awesome-analytics](https://github.com/onurakpolat/awesome-analytics) (analytics),
+  - [staticgen.com](https://www.staticgen.com/)
+  - [staticsitegenerators.net](https://staticsitegenerators.net/) (static site generators).
+- [ ] The `Language` tag is the main server-side requirement for the software - don't include frameworks or specific dialects.
+- [ ] Any license you add is in our [list of licenses](https://github.com/awesome-selfhosted/awesome-selfhosted/blob/master/README.md#list-of-licenses).
 - [ ] You have searched the repository for any relevant [issues](https://github.com/awesome-selfhosted/awesome-selfhosted/issues) or [PRs](https://github.com/awesome-selfhosted/awesome-selfhosted/pulls), including closed ones.
-- [ ] Any category you are creating has the minimum requirement of 3 items. If not, your addition may be inserted into `Misc/Other`.
+- [ ] Any category you are creating has the minimum requirement of 3 items.
+  If not, your addition may be inserted into `Misc/Other`.
 - [ ] Any software project you are adding to the list is actively maintained.
-- [ ] The pull request title is "Add aaa to bbb" for adding software aaa to section bbb. Do not call it "Update README.md".
+- [ ] The pull request title is informative, unlike "Update README.md".
+  Suggested titles: "Add aaa to bbb" for adding software aaa to section bbb,
+  "Remove aaa from bbb" for removing, "Fix license for aaa", etc.