Update README.md

+ added Index for a better view and quicker access to the categories
+ added Features
+ added Limitations
+ added Compatibility
+ added Upgrading
+ added Permissions
~ separated ToDo and already done (updated already done)
~ changed Compile AFWall+ text, some spelling corrections and the fellas should always use a recent software build (e.g. latest javac is 1.9 unstable) so I changed it to 1.8 stable 
-> we should always mention that newer Apis and versions are 99% backwards compatible ... so no need to hang on outdated stuff