Document binary files for no line ending changes

[davidfischer]

The .gitattributes file from #3053 caused some issues on a clean clone of the repository for me. These issues involved modifying the line endings of binary files and manifested themselves in the form of binary files showing as modified in git status with warnings like:

warning: CRLF will be replaced by LF in deploy/wheels/Jinja2-2.7.1-py2-none-any.whl.

At first I was very confused as I would think CRLF endings were only for Windows users. However, I realized that binary files could contain CRLFs. When I looked at the .gitattributes file, it denotes everything as text with Unix line endings. Based on github's useful docs on the subject, I think that documenting binary files will clear this up.

STEPS TO REPRODUCE
For me, a fresh clone of the latest master would immediately show files as modified.

git clone [email protected]:rtfd/readthedocs.org.git
git status

SYSTEM INFORMATION
OS: MacOS 10.12.6
Git: 2.14.1

[ericholscher]

Thanks!