Generate bundle READMEs from a template

[shawnbot]

This is a lazy (yet innovative!) way of keeping the src/**/README.md files up to date. I started trying to address the inconsistencies manually in #858, but quickly found that to be a waste of time. Here's the deal:

• src/README.template.md is a template for the bundle docs with one or more {bundle} placeholders.
• script/generate-bundle-readmes generates a new README.md for each index.scss in the src directory, and replaces the {bundle} placeholder with the directory name. See src/alerts/README.md as an example of the generated output.

If I'm out of town when y'all get around to reviewing this and the template needs tweaking, please go ahead and edit it on this branch and re-run script/generate-bundle-readmes locally, then push your changes to src/**/README.md.

@colebemis: the README files in our src directory are not included on the docs site, so you shouldn't need to worry about this!

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[jonrohan]

Ahh yeah, nice. ✨