Made instructions more clear for those needing Xcode

Firstly, amazing job with this project - it really works.
I had to reinstall a few times to get the capacity right to fit Xcode. Just wanted to save others the trouble by noting the min. needed disk size for Xcode. In my case 50gb was too little, so the exact right amount lies between 50-60gb.

README.md

@@ -140,6 +140,8 @@ docker start abc123xyz567
 - Erase the BIGGEST disk (around 200gb default), DO NOT MODIFY THE SMALLER DISKS.
 -- if you can't click `erase`, you may need to reduce the disk size by 1kb
 
+- (optional) Create a partition using the unused space to house the OS and your files if you want to limit the capacity. (For Xcode 12 partition at least 60gb.) 
+
 - Click `Reinstall macOS`