[chore] Add No Over-Representation clause (#38344)

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#### Description

Adds 'No Over-Representation' clause to the Collector Contrib
maintainers group.

This is inspired by, but different from the [Technical Committee No
Over-Representation
clause](https://github.com/open-telemetry/community/blob/main/tech-committee-charter.md#no-over-representation).
It is different from it in that it does not cover the removal,
resignation or change of employers situations.

---------

Co-authored-by: Juraci Paixão Kröhling <[email protected]>

Author: mx-psi

README.md

@@ -125,6 +125,10 @@ Emeritus Maintainers
 
 Learn more about roles in the [community repository](https://github.com/open-telemetry/community/blob/main/guides/contributor/membership.md).
 
+### No Over-Representation
+
+A community member cannot be promoted to be a Collector contrib maintainer if, after their promotion, the resulting maintainers group has more than one-fourth (25%) of the members affiliated with the same employer. Job changes and similar events might result in over-representation, and no new maintainers from the same company can be promoted until representation is balanced again. In the event of confusion or concern, the OpenTelemetry Collector SIG will defer to the CNCF definition of "same employer".
+
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