"vendor/" Lacks README

[silabs-robin]

Type

• Other.

Steps to Reproduce

cd vendor.
If you don't already know what that directory is, you won't find out in that directory.
Is it documented in a core-specific external repo?
A simple readme file with one sentence would inform either were the documentation is or explain what the directory is and how to use it.
For anyone wishing to use the content of that directory, that directory itself is the most natural place to look.

[Mike Thompson]

Good point @silabs-robin! The same could be said for vendor_lib. I will work with @MarioOpenHWGroup to create a couple of helpful (?) READMEs for these directories.