Document that all agreements should be confirmed on the mailing list

**Issue Type**
[x] Bug
[ ] Feature Request

**Priority**
[ ] Minor
[x] Major
[ ] Critical
[ ] Blocker

**Environment**
* OS: all
* Browser: any

**Description**

From the query I raised in [this thread on lists.apache.org](https://lists.apache.org/thread.html/f73c9e2a8fd1ebe48c53cbe2975ad6908df5f322f47ce6e0ae011647@%3Cdev.cordova.apache.org%3E):

I recently learned the following from @purplecabbage in [this response](https://lists.apache.org/thread.html/e10286fc9207c2388b734af910f23b97db5deac34a7cd813b67c849d@%3Cdev.cordova.apache.org%3E) to [the thread](https://lists.apache.org/thread.html/f73c9e2a8fd1ebe48c53cbe2975ad6908df5f322f47ce6e0ae011647@%3Cdev.cordova.apache.org%3E):

> We actually used to be a lot stricter with this, but I think the one rule
> is, was, and for always should be "If it didn't happen on the list, it
> didn't happen."

I think this should me made much more clear to new committers (and PMC members), new contributors, and new app developers using Cordova in general.

Further suggestions from @purplecabbage in [this later reply](https://lists.apache.org/thread.html/9eec9d482d3f5eb42d8d1a3b803921bf4af1c26098495139aed275d9@%3Cdev.cordova.apache.org%3E) to [the same thread](https://lists.apache.org/thread.html/f73c9e2a8fd1ebe48c53cbe2975ad6908df5f322f47ce6e0ae011647@%3Cdev.cordova.apache.org%3E):

I think we could do a better job with our guide at [1] which we need to
update anyway with all the github issues going on now.
The Apache site also has some really good text on lazy-consensus [2]

Ultimately though, I think it comes down to all of us just doing it. If
someone shows up on this list, and we are all following a clear process, it
is probably pretty easy to jump in.

[1] https://cordova.apache.org/contribute/contribute_guidelines.html
[2] http://community.apache.org/committers/index.html

There are some other responses in [the thread](https://lists.apache.org/thread.html/f73c9e2a8fd1ebe48c53cbe2975ad6908df5f322f47ce6e0ae011647@%3Cdev.cordova.apache.org%3E), along with some links into apache.org, that I think are important and should be better documented as well.

I hope I get a chance to contribute the needed clarification (someday). I am raising this issue in hopes that it will not be forgotten.

Start with the contribute guidelines linked in the issue and review the Apache committers guidance and referenced mailing-list thread. Identify the project-process agreements that need clarification for committers, contributors, and app developers, then update the relevant documentation so the mailing-list confirmation process is explicit.

Written by the indexing model from the issue text.