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Readme: Mention Sentry integration in feature list
Readme: Various style fixes and text adjustments
Readme: Fix reStructuredText markup

Sleepy me somehow managed to confuse Markdown and reStructuredText
markup with each other. Yes, I'm a dummy. :)
Further update the readme

Mainly formatting changes, update/change feature list, installation
instructions and add an example of a hello world plugin.
Merge pull request #151 from zoni/readme

Readme update