Update Contributing.md and ReadMe.md

[theecrit]

What are you submitting?

Place an x in the appropriate bracket:

• Bug (something's broken)
• Feature request or improvement
• Content update
• Question
• Other

Description

Currently, the ReadMe.md and Contributing.md files include some inaccuracies and omissions.

Contact info

• ReadMe.md contact section should be replaced with:

Contact the team

• OpenOakland leadership can be reached at steering@openoakland.org.
• Everyone is welcome to join our weekly meetups.

• Contributing.md

Misc.

• Remove "Additional project information" section, as it's out of date.
• Add instructions for updating nav menu via _config.yml and navigation.yml (plus note that seeing changes requires rebuilding the site locally).
• Add instructions for calendar events:

Managing the /calendar section

• Events listed on the embedded Google Calendar are tied to the administrator@openoakland.org account. To add or edit an event, send a request to steering@openoakland.org.
• To add an event to the Past Events section, follow the commented instructions in /src/_data/calendar.yml.

Thank you!