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

[inkydragon]

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.