Fixed example in authentication docs
Adding link to documentation in README.rst

Just adding a link to the documentation at python-requests.org
Merge pull request #1342 from mexicat/master

Fixed example in authentication docs
Merge pull request #1352 from nicoddemus/docs-link-on-readme

Adding link to documentation in README.rst
Prevent a KeyError when there's no link headers
Merge pull request #1355 from sigmavirus24/fix_links

Prevent a KeyError when there's no link headers