header/footer: use absolute URLs for links to CONTRIBUTING.md, LICENSE, non-free.md

- ref. https://github.com/awesome-selfhosted/awesome-selfhosted/issues/1038
- ref. https://github.com/awesome-selfhosted/awesome-selfhosted-data/tree/master/markdown

README.md

@@ -4,9 +4,9 @@
 
 Self-hosting is the practice of hosting and managing applications on your own server(s) instead of consuming from [SaaSS](https://www.gnu.org/philosophy/who-does-that-server-really-serve.html) providers.
 
-This is a list of [Free](https://en.wikipedia.org/wiki/Free_software) Software [network services](https://en.wikipedia.org/wiki/Network_service) and [web applications](https://en.wikipedia.org/wiki/Web_application) which can be hosted on your own server(s). Non-Free software is listed on the [Non-Free](non-free.md) page.
+This is a list of [Free](https://en.wikipedia.org/wiki/Free_software) Software [network services](https://en.wikipedia.org/wiki/Network_service) and [web applications](https://en.wikipedia.org/wiki/Web_application) which can be hosted on your own server(s). Non-Free software is listed on the [Non-Free](https://github.com/awesome-selfhosted/awesome-selfhosted/blob/master/non-free.md) page.
 
-See [Contributing](.github/CONTRIBUTING.md).
+See [Contributing](#contributing).
 
 --------------------
 
@@ -2185,7 +2185,7 @@ _See also: [Wikimatrix](https://www.wikimatrix.org/), [Wiki Engines - WikiIndex]
 
 ## Contributing
 
-Contributing guidelines can be found in [.github/CONTRIBUTING.md](.github/CONTRIBUTING.md).
+Contributing guidelines can be found in [.github/CONTRIBUTING.md](https://github.com/awesome-selfhosted/awesome-selfhosted/blob/master/.github/CONTRIBUTING.md).
 
 ## Authors
 
@@ -2193,4 +2193,4 @@ The list of authors can be found in the [AUTHORS](AUTHORS) file.
 
 ## License
 
-This list is under the [Creative Commons Attribution-ShareAlike 3.0 Unported](LICENSE) License.
+This list is under the [Creative Commons Attribution-ShareAlike 3.0 Unported](https://github.com/awesome-selfhosted/awesome-selfhosted/blob/master/LICENSE) License.