CMIP6 evaluation activity update to guide page

[durack1]

@taylor13 here is a placeholder issue to grab this idea, obviously, we'll need to pad out the table below considerably.

Under https://pcmdi.llnl.gov/CMIP6/Guide/dataUsers.html#8-cmip6-organization-and-governance we have "evaluation activities" itemised, this should include:

Activity	PI	URL	github/repo
PCMDI Metrics Package	Peter Gleckler	PMP	https://github.com/PCMDI/pcmdi_metrics
ESMValTool	Veronika Eyring	https://www.esmvaltool.org/	https://github.com/ESMValGroup
?	?	?	?

[taylor13]

I might omit the PI's; perhaps include a terse description?

[durack1]

Activity	URL	github/repo	Info
PCMDI Metrics Package	PMP	https://github.com/PCMDI/pcmdi_metrics	a cool package
ESMValTool	ESMValTool	https://github.com/ESMValGroup	another cool package
?	?	?	coolest?

[znicholls]

One for the list (shameless self promotion): netCDF-SCM, https://gitlab.com/netcdf-scm/netcdf-scm (also https://doi.org/10.5281/zenodo.4536523) Simple wrappers for processing netcdf files for use with simple climate models