Add guidance and help SIGs create a SIG charter

[mx-psi]

We have a GC charter and a TC charter, but we don't have a similar document for SIGs like Kubernetes does. Newer projects have the project description, which some SIGs use as a reference point, but they are not frequently updated and there is some information that may not be readily available for users.

Having this document would be beneficial for:

• Helping contributors understand the scope of each SIG and what it works in
• Helping contributors and companies understand what the contributor ladder looks like and what additional responsibilities triagers/approvers/maintainers have

The lowest hanging fruit IMO would be to create a template and work with a few SIGs that are interested to test this out.