add recommendations for MySQL installation

[sealocal]

Reason for change:

The README says to download MySQL from dev.mysql.com, but Mac users should use homebrew instead.

$ homebrew install mysql

Otherwise, when you run $ rake you might experience an error similar to this:

rake aborted!
dlopen(/Users/username/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/mysql2-0.3.11/lib/mysql2/mysql2.bundle, 9): Library not loaded: libmysqlclient.18.dylib
Referenced from: /Users/username/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/mysql2-0.3.11/lib/mysql2/mysql2.bundle
Reason: image not found - /Users/username/.rvm/gems/ruby-1.9.3-p194@rails3tutorial2ndEd/gems/mysql2-0.3.11/lib/mysql2/mysql2.bundle

[rap1ds]

Looks good, thanks!

[sealocal]

No problem!

[Mikaela]

What if you use MariaDB or some other MySQL compatible replacement?

[gnomet]

@Mkaysi We haven't tested any other mysql compatible solutions in the core team, and are not aware of others reporting of trying them yet either. If you try, it would be great to hear if it works or not. And if you want to add a comment about that to installation instructions, that would be a great addition too. :)