Add mention about gelf.extensions.logging

[tomap]

Thanks for contributing to Logstash! If you haven't already signed our CLA, here's a handy link: https://www.elastic.co/contributor-agreement/

[tomap]

I do not seem to be able to sign the agreement. Page seems broken

[tomap]

I signed it

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
16e2132

Please, read and sign the above mentioned agreement if you want to contribute to this project

[karenzone]

jenkins test this please

[karenzone]

@tomap The CLA check is still failing. Sometimes this happens when the email address for the commit and the email address you used to sign the CLA are different. Please take a look and let me know.
Thanks! We are looking forward to checking out your contribution.

[karenzone]

jenkins test this please

[karenzone]

jenkins test this please

[karenzone]

@tomap The CLA check is still failing. Sometimes this happens when the email address for the commit and the email address you used to sign the CLA are different. Please take a look and let me know.
Thanks! We are looking forward to checking out your contribution.

[karenzone]

jenkins test this please