Info about getting set up using composer and MAMP

Added by request of Gábor Novoszádi from community feedback and thanks to the gist from @kkirsche. I have supplied a link to the gist as it is well written, with permission we may look to rework for direct inclusion at some point :)
Merge pull request #3010 from camfindlay/patch-3

Info about getting set up using composer and MAMP