Add steps for how to add a new externel dep for julia

We've added MMTK as a new dep.
So, it's a good time to write down those steps.
I'm going to document the overall process, give a link to the MMTK pr as a reference.

- create and publish a new `mmtk_jll` pkg
- add new dep to julia with source build
- add new dep to julia with pre-build jll
- setup ci (In buildkite repo), build julia with MMTK
- run PkgEval with julia+MMTK
how to use PkgEval bot

And How to update an exist dep to new version.

- update jll
- update version number / commit hash in julia repo
- update checksum

And where to ask for help.