add instructions on how to update embedded ots

... when a new version is released upstream.

It should be as easy as changing the "version" and "sha256" values in `setup.cfg` file:

https://github.com/googlefonts/ots-python/blob/dc1c9251004af0e3c99a8b08c13d3f9250cb448d/setup.cfg#L1-L6

commit that change, create a git tag with the same version number (always keep a leading `v`, e.g. `v7.1.9` in the git tag name), and push the tag. The bots will do the rest.