howto_contribute: changing UX? then add to CHANGELOG.rst. and maybe docs

I've been forgetting to update the CHANGELOG.rst file. Maybe I'll remember it if I add a mention to the docs... either because it's doc'd or because of memories of this public confession.

Bugs closed: 397

Reviewed at https://rbcommons.com/s/twitter/r/992/
remember to update the CHANGELOG

Reviewed at https://rbcommons.com/s/twitter/r/993/
When a directory contains both a java_library and junit_test def, make sure the IDE understands this is a test pth, not a lib path

In our repo we have projects with BUILD files with 2 targets like:

```
java_libary(name='lib', ...)
junit_test(name='test', ...)
```

Sometimes, the ide_gen code saw the 'java_library' target first and considered that a regular source path, not a test source path, which causes tests to break in the IDE if they depend on libraries only defined in another test source path.  In project.iml, we would see the path defined with is_test="False"

This is just a workaround, as ide_gen still might not always add test folders properly:  If there is a java_library defined in a BUILD file under /src/test/java but no  junit_test, ide_gen mistakenly thinks this is plain source path, not a test source path.

Testing Done:
Re-ran goal idea on our test targets, the folders containing both tests and libraries that were previously 'blue' (regular source paths) are now green as they should be.

Bugs closed: 551

Reviewed at https://rbcommons.com/s/twitter/r/996/
initial draft of publishing a pants test infra package to facilitate external pants plugin testing

Bugs closed: 539

Reviewed at https://rbcommons.com/s/twitter/r/997/