Adding a link to our Mercurial tutorial to the index page used on the main web site
Placeholder TODO
Add screenshoots, still need explanations
Add images for PR comments
Add comments to explain the images on code review
Clarify the flow
Fix wrong pull origin in images
Update filenames, since they are instructions to GitHub, not git
Put the comment icon inline
Remove comment
Updated diagrams in git/01-backup
Fixed SVGs
Describing the bug, discussion, enhancement, and lesson labels.

This PR:

1.  Adds descriptions of the `bug`, `discussion`, `enhancement`, and `lesson` labels.
2.  Puts all label descriptions in alphabetical order.
Merge pull request #693 from gvwilson/describing-labels

Describing the bug, discussion, enhancement, and lesson labels.
Add learning objective to Intro to Version Control with Mercurial.
Add explanation of `hg update` after `hg pull`.
Merge pull request #696 from douglatornell/hg-collab-pull-update

Add explanation of `hg update` after `hg pull`.
Merge pull request #695 from douglatornell/hg-intro-objective

Add learning objective to Intro to Version Control with Mercurial.
Merge pull request #681 from sjpfenninger/git-01-diagrams

Improved diagrams in git intro lesson
Merge pull request #667 from luizirber/lesson-improvement/code-review

Lesson improvement: code review