feat(README): add an API versioning section

Based on a question on the mailing list. Add a section about API
versioning.
Merge pull request #421 from philips/README-api-versioning

feat(README): add an API versioning section
Merge pull request #422 from dsoprea/master

Add documentation for enumerating sorted, in-order keys.
docs(README.md) fix wrong index value in lock section
Merge pull request #424 from xiangli-cmu/fix_lock_doc

docs(README.md) fix wrong index value in lock section
fix(README): remove prevValue references in the README

We plan on adding a PrevNode field in the Response object but it hasn't
been needed thus far. Discussion was here:

http://thread.gmane.org/gmane.comp.distributed.etcd/56

Fixes #428
chore(release_version): remove this unused file

Fixes #425
Weird grammar, not sure I fixed it well enough.
Merge pull request #431 from btipling/master

Weird grammar, not sure I fixed it well enough.
Merge pull request #430 from philips/remove-old-release-version

chore(release_version): remove this unused file
Merge pull request #429 from philips/README-remove-prevValue

fix(README): remove prevValue references in the README
Added docs for directory deletes.
Update -X parameter to have consistent spacing in docs.