Add glossary extension

[andrewpbray]

What does this PR do?

Adds a ref to the glossary extension, a lightweight implementation that copies blocks of content from across many docs and adds them to a different doc as a glossary.

• glossary - Collect blocks of content across many documents and write them into a glossary.

Requirements for your Awesome item list

• Only has awesome items. Awesome lists are curations of the best, not everything.
• Does not contain items that are unmaintained, has archived repo, deprecated, or missing docs. If you really need to include such items, they should be in a separate Markdown file.
• Entries have a description, unless the title is descriptive enough by itself. It rarely is though.
• Has consistent formatting and proper spelling/grammar.
  - The link and description are separated by a dash.
  Example: - [AVA](…) - JavaScript test runner.
  - The description starts with an uppercase character and ends with a period.
  - Consistent and correct naming. For example, Node.js, not NodeJS or node.js.
• Doesn't use hard-wrapping.
• Your item(s) fit into an existing sections. (if you think a new section is needed, please explain why your item does not fit into an existing one and what would be the interest of your new section.
• Add your new item to the bottom of the list in a section.
• If a duplicate item exists, discuss why the new item should replace it.
• Check your spelling & grammar.
• The item must follow this format:

  - [item name](https link) - Description beginning with capital, ending in period.
  

• Be sure to have read the contributing guidelines.

[mcanouil]

Thanks for the PR!
Does it work for any formats? I could not find anything in your README stating the compatibility.

[andrewpbray]

Good question. Let me do some tests. I'll put the results in the README then ping you here.

[andrewpbray]

@mcanouil README has been updated with a compatibility section.

[mcanouil]

Thanks, merging now!