Update README.md with installation information

Squashed commit of the following:

commit 09093f247732543f41630310b154ec3f484267af
Author: Marcus Hightower <mhightower@singlehop.com>
Date:   Thu Apr 3 18:22:24 2014 -0500

    Fix markdown table

commit 35eee2f7014000578a4e258e25564221aac824ab
Author: Marcus Hightower <mhightower@singlehop.com>
Date:   Thu Apr 3 18:19:35 2014 -0500

    Explain the dist feature of Codeception and the way it used here.

commit 86440071e45286a3d4f804bac20cd0a7de11a752
Author: Marcus Hightower <mhightower@singlehop.com>
Date:   Thu Apr 3 18:05:56 2014 -0500

    Better example note

commit 92f6e1856bf38cdc895a5c061dd735d8e3b39626
Author: Marcus Hightower <mhightower@singlehop.com>
Date:   Thu Apr 3 17:50:21 2014 -0500

    Update docs and require version of PHP.