rules for when build string includes build hash

[electronwill]

from MichaelS: The rules for when it shows up are at https://github.com/conda/conda-build/releases/tag/3.1.0 and need to be added to the docs

• package is an explicit dependency in build, host, or run deps
• package has a matching entry in conda_build_config.yaml which is a pin to a specific version, not a lower bound
• that package is not ignored by ignore_version

OR

• package uses {{ compiler() }} jinja2 function

All other packages will no longer have hashes. The takeaway message is that hashes will appear when binary compatibility matters, but not when it doesn't.

(moved from https://github.com/ContinuumIO/docs/issues/1809 )

[rrigdon]

Should go here most likely: https://docs.conda.io/projects/conda-build/en/latest/concepts/package-naming-conv.html#index-2

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