maint: add MIT license badge to README.md.
maint: add MIT license badge to README.md.

* README.md: Add MIT license badge.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

snegovick/luaposix,ardje/luaposix,snegovick/luaposix,rrthomas/luaposix,jimmyharris/luaposix,rrthomas/luaposix,jimmyharris/luaposix,luaposix/luaposix,ardje/luaposix