New glyph: Ṁ (Mdotaccent)

Hi there,

I was missing the ṁ and Ṁ glyphs (mdotaccent, m with dot above) for one of my projects recently.
The lowercase ṁ glyph got added in the `post-4.005-fixes` branch, but the corresponding uppercase glyph is still missing.

Please let me know what the procedure is to get this glyph included for future releases.

I've already forked the development branch and added the glyphs to all masters, including small caps for the Roman masters.
If you'd like I'd be happy to submit a PR.

Cheers