Update maintainer nomination process (jaegertracing#6100)

## Which problem is this PR solving?
- The existing process requires the use of the mailing list for
nominations, but we don't use that list very much
- It also requires disclosing the email address

## Description of the changes
- Allow using GitHub issue as alternative to the mailing list
- Do not require disclosure of the email address

Signed-off-by: Yuri Shkuro <[email protected]>

Author: yurishkuro

GOVERNANCE.md

@@ -26,10 +26,9 @@ To become a maintainer you need to demonstrate the following:
 
 A new maintainer must be proposed by an existing maintainer by sending a message to the
 [[email protected]](https://groups.google.com/forum/#!forum/jaeger-tracing)
-mailing list containing the following information:
+mailing list, or by opening an issue on GitHub, containing the following information:
 
-  * nominee's first and last name,
-  * nominee's email address and GitHub user name,
+  * nominee's first and last name and GitHub user name,
   * an explanation of why the nominee should be a committer,
   * a list of links to non-trivial pull requests (top 10) authored by the nominee.