Add CODEOWNERS file to automatically request reviews.

Summary:
Recently announced: https://github.com/blog/2392-introducing-code-owners

These are non-blocking for the time being.
Closes #14897

Differential Revision: D5385189

Pulled By: hramos

fbshipit-source-id: 711827573f85577b70f3627d66a1f9f00fe7f90d

.github/CODEOWNERS

@@ -0,0 +1,6 @@
+docs/* @hramos
+blog/* @hramos
+website/* @hramos
+website/showcase.json @hramos
+package.json @hramos @ericnakagawa
+website/package.json @hramos @ericnakagawa