draft "how to set up a working group"

[NickleDave]

as discussed at 2022-01-28 meeting https://docs.google.com/document/d/19cgBPbbjk7P7FMSag_Ld3Pcd28cSajTypZTZ_b47VXc/edit#heading=h.msq7ung7u1b2

with involvement of current working groups of course (as suggested by @cmaimone)

[NickleDave]

• [ ] be sure to update "getting_started/getting_started.md" on US-RSE site after this is written, as discussed in https://github.com/USRSE/usrse.github.io/pull/666