add ReadTheDocs build status badge

[rutsky]

Currently documentation build result status is not displayed anywhere.
Lets show it at least as badge.

[rutsky]

Note that badge is linked on page with recent docs builds, not on documentation web site.

[codecov-io]

Current coverage is 84.11%

Merging #2027 into master will increase coverage by +0.01% as of d17d779

@@            master   #2027   diff @@
======================================
  Files          330     330       
  Stmts        32049   32049       
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit          26956   26957     +1
  Partial          0       0       
+ Missed        5093    5092     -1

Review entire Coverage Diff as of d17d779

---

Uncovered Suggestions

1. +0.10% via ...ot/status/builder.py#381...414
2. +0.09% via ...ot/status/builder.py#453...481
3. +0.08% via ...dbot/changes/mail.py#478...505
4. See 7 more...

Powered by Codecov. Updated on successful CI builds.