Share SciTools/iris#6386 `MANIFEST.in` improvements via templating?

[scitools-ci]

SciTools/iris#6386 (by @pp-mo) includes changes to MANIFEST.in. This file is not currently templated, but its parent directory suggests it may be a good candidate. Please either:

• Action this issue with a pull request adding a template file to templates/.
• Close this issue if the file is not a good candidate for templating.

[trexfeathers]

Yes, but we need a MANIFEST.in template first! #42