Add howto submit presentations to p.cncf.io (#611)

Signed-off-by: Jorge O. Castro <[email protected]>

Author: castrojo

website/content/resources/how-to/submit-presentations.md

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+---
+title: Submit Presentations to presentations.cncf.io
+---
+
+[presentations.cncf.io](https://presentations.cncf.io/) exists for projects and other contributors to index their talks in a searchable portal. Presentations can be submitted by filing a pull request in the [presentations GitHub repository](https://github.com/cncf/presentations). 
+
+Contributors that can review and accept pull requests are also welcome!