v0.2.1 — spread polls, and publication tidy-up

A politeness release. No sensor values change and nothing needs
reconfiguring.

Polls no longer land on the dot

Every installation of this integration was hitting the API at exactly
08:00:00, 11:00:00 and so on. Each installation now picks a fixed offset of up
to 15 minutes and keeps it, so yours might poll at 08:06, 11:06, 14:06 every
day.

The offset is stable for your installation, not re-rolled each time —
spreading load is about not colliding with other people's installations, so
a fixed offset achieves that while "when does it poll?" still has an answer.
Two accounts in one household get two different offsets. The offset is always
added after the slot, so nothing ever fires before 08:00.

It also keeps polls off the top of the hour, where every cron job and every
other integration already is.

Documentation and repository

• The README, the release notes and the test fixtures were carrying the test
  account's real points balance as the worked example. Replaced throughout
  with a round illustrative figure. The v0.2.0 release note has been edited
  too.
• Issue templates, which ask for versions and — more importantly — remind you
  to redact your address, telephone number, employee numbers and email before
  pasting any log. This integration never logs those, but other components'
  debug output can, and entity IDs can carry your name.
• README badges, "My Home Assistant" buttons for adding the repository and
  starting the config flow, and a Contributing section.
• The README now says plainly that this is not in the HACS default store, so
  adding it as a custom repository is the expected route rather than a
  workaround.