Add MacOS specific installation notes

[tjtelan]

Most of this issue is to address the bad experience I had with trying to compile orbital on MacOS 11.1.

  = note: ld: library not found for -lpq
          clang: error: linker command failed with exit code 1 (use -v to see invocation)
          

error: aborting due to previous error

error: could not compile `orb`

Which I fixed by installing postgres via homebrew (Installing libpq did not solve).

brew install postgres

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I also needed to install mariadb via homebrew in order to install diesel_cli for the database migrations.

brew install mariadb