add annotations documentation.

Fix regex, mention reserved labels.

Point out README.md build section from tutorial.

Add some FAQs.

- FAQ about timezone.
- FAQs about implementation.
- Fix swapped issue URLs.
- Add section about possible hash collisions.
- Mention `GOMAXPROCS`, especially important for Go beginners.
- Mention possible storage optimization for histograms.

Clarify that to_nearest may be a fraction in round().

Document rounding functions.

Add collectd. Reorder based on likely popularity.

Add staleness and clients/exporters

Add FAQ section on stability guarantees.

Improve documentation about storage.

Also add related FAQs (including the ZFS bug).

Add roadmap documentation.

Update 'random-example' section.

1) Use https to clone 'client_golang' repository, like the 'prometheus'
   repository.
2) Update build instructions to specify 'example_random' target instead
   of 'random_example'. I updated the sample configs, too.

Use a protocol-relative URL to load Google Fonts

Add Docker install instructions

promdash is more useful for most users