Update release documentation

[neufeldtech]

Resolves #368

[neufeldtech]

Note: When testing these instructions, I came across a problem with the proposed workflow of creating a pull request for updating the release notes and preparing the release...

When I did this, because github forced me to do a squash and merge, then there was a mismatch between the commit that landed on master and the tag that I had previously created locally.

Options for this workflow are:

• Prepare release and always push release commit directly to master, and push tag directly afterwards
  OR
• Create the tag after the merge commit lands on master - however, then this also means that we can't really run the 'make test-release' workflow locally because then it would cause us the need to update our local tag again.

@miry when you're back from vacation, let's chat on this some more

[miry]

@neufeldtech Thank you for the doc ❤️

[neufeldtech]

Hmmm, I might have a flaky test....

=== RUN   TestProxyMetricsReceivedSentBytes
    metrics_test.go:36: expected: [toxiproxy_proxy_received_bytes_total{direction="upstream",listener="localhost:0",proxy="test",upstream="upstream"} 5 toxiproxy_proxy_sent_bytes_total{direction="upstream",listener="localhost:0",proxy="test",upstream="upstream"} 5] got: []

This test was added in my prometheus metrics PR #366