Explain the produced Jars to avoid missuses
Explain the produced Jars to avoid missuses

I spent some hours to understand that I was trying to execute the wrong Jar:  
I was wondering for hours what was wrong in the building process about produced `build/libs/sbe-all-1.7.0.jar` without `META-INF/MANIFEST.MF`.

I think I am not alone because it was also the issue #324

Instead of creating a new issue, I propose in this commit to provide some documentation in the README.md about produced Jars, and a warning to prevent using the wrong Jars.
real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,marksantos/simple-binary-encoding,real-logic/simple-binary-encoding,marksantos/simple-binary-encoding,PKRoma/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,real-logic/simple-binary-encoding,PKRoma/simple-binary-encoding,marksantos/simple-binary-encoding,PKRoma/simple-binary-encoding,marksantos/simple-binary-encoding,marksantos/simple-binary-encoding,PKRoma/simple-binary-encoding,PKRoma/simple-binary-encoding,marksantos/simple-binary-encoding,marksantos/simple-binary-encoding,PKRoma/simple-binary-encoding,real-logic/simple-binary-encoding,marksantos/simple-binary-encoding