Improve README with towncrier instructions.

[MihaiTabara]

Ditching this PR as README fixes were picked up by @nthomas-mozilla in https://github.com/mozilla-releng/beetmoverscript/pull/158 PR. I'll follow-up with a separate PR to take into account what @tomprince suggested - https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/

[coveralls]

Pull Request Test Coverage Report for Build 557

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.913%

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Totals
Change from base Build 554:	0.0%
Covered Lines:	455
Relevant Lines:	460

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💛 - Coveralls