filippo: create github.txt

Couldn't wait testing the new shiny GitHub PR integration.

Change-Id: Id6f732f66a48b4d842dfef359783a9f7981faefd
GitHub-Last-Rev: 23cf5d21d0
GitHub-Pull-Request: golang/scratch#17
Reviewed-on: https://go-review.googlesource.com/92875
Reviewed-by: Filippo Valsorda <hi@filippo.io>

filippo/github.txt

@@ -0,0 +1,3 @@
+Straight from the GitHub web editor!
+
+This will be awesome for occasional contributors, thanks @andybons and team!