Missing contributing guidelines

## Describe the bug in a sentence or two.
The target of the link:
https://github.com/cloudinary/cloudinary_java/blob/43d8e9ac0c7f073017d9c4b566c9566dea95f205/README.md?plain=1#L130
does not exist.

## Issue Type (Can be multiple)
[ ] Build - Can’t install or import the SDK
[ ] Performance - Performance issues
[ ] Behaviour - Functions aren’t working as expected (Such as generate URL)
[ ] Documentation - Inconsistency between the docs and behaviour
[x] Other (Specify)