Update CLA check text in maintainers guide

[agcolom]

This section needs to be updated as the CLA check is now automatic.

[scottgonzalez]

I'd prefer if someone other than me made this change to make sure others are aware of what's actually going on and how to deal with issues.

[agcolom]

Some notes:

• This should mention the label being applied
• What happens when the CLA is signed more than once (i.e. does it check for first match, or array of matches)
• Does it/Can it check again after a new signature
• Does it/Can it check again after a change of name in git config