Define Spec API best practices

[vishh]

To make contributions easier, as we come up with best practices for the Spec, we should document them.
For example, we decide to use pointers for all fields that need to provide a "do not touch" option.

[wking]

+1 from me 1 :).

 Subject: Recording style and policy in opencontainers/specs
 Date: Wed, 9 Sep 2015 11:09:29 -0700
 Message-ID: <20150909180929.GS18140@odin.tremily.us>