doc: self nomination to diagnostics working group

I'd like to nominate myself to this working group.
I have been attending the last few sessions;
and I am interested in its theme and content.
I have contributed to the best practices content,
and plan to contribute more.

PR-URL: #379
Reviewed-By: Stephen Belanger <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Gerhard Stöbich <[email protected]>

README.md

@@ -61,6 +61,7 @@ See the [foundation calendar](https://nodejs.org/calendar) for meeting times.
 - [@cjihrig](https://github.com/cjihrig) - Colin Ihrig
 - [@Flarna](https://github.com/Flarna) - Gerhard Stöbich
 - [@gireeshpunathil](https://github.com/gireeshpunathil) - Gireesh Punathil
+- [@harshithakp](https://github.com/harshithaKP) - Harshitha K P
 - [@hashseed](https://github.com/hashseed) - Yang Guo
 - [@hekike](https://github.com/hekike) - Peter Marton
 - [@Hollerberg](https://github.com/Hollerberg) - Gernot Reisinger