Add Slack link to README

Invite users and potential contributors to join us on Slack as well as
IRC.

Add docs explaining how to test the client on a real mobile device

Use angle brackets to indicate that HOSTNAME is a placeholder rather
than a dollar-sign to avoid confusion with the `$HOSTNAME` env var.

 * Add note about possible need to add `.local` or `.home` suffix or use
   IP address instead of hostname.
 * Use title case for heading.

Merge pull request #333 from hypothesis/fix-annotating-sites-with-broken-Function.bind-polyfills-take-3

Fix annotating sites with broken function.bind polyfills take 3

Add a lint rule to ensure that new developers do not use `bind` in
annotator/ code. This only applies to first-party code. Third-party code
used in the annotation layer is still at-risk of being broken by old
`bind` polyfills.

Don't call the parent browsing environment's Function.bind() function
from the src/annotator code.

Some parent pages modify Function.protoype.bind(), for example replacing
it with their own broken bind() polyfill, and this causes our code to
crash.

Work around this by just not using bind().

Fixes #245

Translate annotation-sync.coffee to JavaScript

Use new endpoint for creating an annotation flag

This function has no tests and I don't know what it does but sometimes
`undefined` is passed to it as the `annotations` arg and it crashes, so
fix that.

This was a CoffeeScript-generated variable.

This was a CoffeeScript-generated variable.

These were generated by CoffeeScript (due to its implicit returns) but
don't actually appear to be necessary.

Minimal changes to the CoffeeScript-generated annotation-sync.js to
stop `make lint` complaining about it, while still having `make test`
pass.

annotation-sync.js is the unmodified output of
`coffee -bc src/annotator/annotation-sync.coffee`.

Fix Mobile Safari Scrolling

* master:
  Add tests for sidebar closing when user clicks or taps in document
  Fixing tapping anchored annotations not opening sidebar
  Fixing sidebar not closing on outside content click issue
  eslint: Ignore docs/_build
  Update ESLint to v3.18.0

Since the `<thread-list>` is now contained within a scrollable element
which is not `document.body`, it needs to scroll that element rather
than the window in order to scroll the element into view.

 * Remove the hard-coded selector used to find the scroll root and
   instead search for the closest ancestor element to the
   `<thread-list>` whose computed style has `overflowY` set to `scroll`.
 * Set the scroll root Element's `scrollTop` to scroll the thread list
   instead of calling `window.scroll`
 * Compute the maximum scroll offset based on the `scrollTop` and
   `scrollHeight` properties of the scroll root element instead of
   using `document.body.scrollHeight`.
 * Update tests for `<thread-list>` to check for the scrollable
   ancestor being scrolled instead of the window.

Fixing sidebar not closing on outside content click issue

Fixing tapping anchored annotations not opening sidebar

Testing the client on a mobile device currently requires several steps
which will be non-obvious to new developers. Add documentation to guide
users through the process.

eslint: Ignore docs/_build