[documentation request] installation procedure for MacOS

[KageKirin]

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Hi,

I should probably provide this rather as pull request, but here are the steps necessary to install reallymine on MacOS.

1. install homebrew
2. install go through brew: brew install go
3. get go's path: brew info go. this should write the path, e.g. /usr/local/Cellar/go/1.16.2/
4. install reallymine through go get: GOPATH=/usr/local/Cellar/go/1.16.2/ go get github.com/andlabs/reallymine
5. check that reallymine is in the bin folder: ls /usr/local/Cellar/go/1.16.2/bin/
6. to run reallymine, add /usr/local/Cellar/go/1.16.2/bin/ to your $PATH or use the absolute path as /usr/local/Cellar/go/1.16.2/bin/reallymine.

Cheers.