add comments

[mohammaddvi]

I added some comments in composable and flowable activity. I have plan to add some comments for whole repository If you like them or nor or any comment please be easy to tell me

[mwshubham]

This is a great initiative but Observable.java class is well document and one can review the source code to understand the operator working. Adding additional comments might not be that helpful. cc: @amitshekhariitbhu