github: added a conda forge release status badge

[juarezr]

This PR has the objective of providing visibility to Conda Forge continuos delivery pipeline

Changes

1. Added new badge pointing to conda-forge/petl-feedstock automerge action status
2. Improved badges alt descriptions
3. Release v1.7.7 in Conda Forge

Checklist

Use this checklist for assuring the quality of pull requests that include new code and or make changes to existing code.

• [ ] Source Code rules apply:
  • [ ] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [ ] All changes are documented in docs/changes.rst
• [ ] Versioning and history tracking rules apply:
  • [X] Using atomic commits when possible
  • [ ] Commits are reversible when possible
  • [ ] There is no incomplete changes in the pull request
  • [ ] There is no accidental garbage added in source code
• [ ] Testing rules apply:
  • [ ] Tested locally using tox / pytest
  • [ ] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [ ] Work in progress / Further changes needed
  • [X] Ready to review
  • [ ] Ready to merge

[coveralls]

Coverage remained the same at 91.759% when pulling 1325d20806b86ff1880bed17d8302cfce918e755 on juarezr:add-conda-forge-badge into 0373331c007eb11a1d50a68331c7b88e616725b1 on petl-developers:master.