Add a Development section to the README with tips for contributors

[todo[bot]]

Because of the layout of the package (single "plugin", `boot.py`, and the result of the code in `src/`) it's recommended to use https://packagecontrol.io/packages/AutomaticPackageReloader so that changes you make during development get picked up without needing to restart ST. Try to format the Python code using https://github.com/psf/black. Be mindful of ST API versions https://www.sublimetext.com/docs/api_reference.html. We support ST3 (build 3211+) for now, but may go ST4 exclusive in the future.

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https://github.com/frou/st3-gitblame/blob/d30f1ae616686f4c9a22b4b28e9a38d99c6a5bb2/README.md#L50-L55

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This issue was generated by todo based on a todo comment in d30f1ae616686f4c9a22b4b28e9a38d99c6a5bb2. It's been assigned to @frou because they committed the code.