Add CONTRIBUTING.md

[sohang3112]

Document these instructions for how to build this code in CONTRIBUTING.md so that any contributors can easily get started:

J android is not built with eclipse or any ide, instead it uses the command line tool ant. To build it, type

$ ant release

and then sign the apk, I use jarsigner to do it

ant had been decommissioned by google android toolchain for quite a long time already. It needs some hacking in order to build with ant now.

J android is intended to be able to run on old devices, so that deprecated api are still used. All warnings are ignored.

Project files for android studio or eclipse, etc are welcome. Sorry I use command line tools and being ignorant of IDE.

Originally posted by @bilam in #3

[sohang3112]

Additionally since ant is old & decommissioned, docs should mention where it can be downloaded, and exactly what bit of hacking is needed to build code with it now.

[bilam]

jandroid can be built using either ant or gradle. see readme and readme-ant.
see also the yml file under the .github folder for automatic building using github action,

gradle is recommended for new users. I prefer ant just because I have used it to build jandroid for a decade.