Add robots.txt
Add robots.txt

Summary: Prevent bots from inflating metrics.

Test Plan:
Verified file is served correctly in integration. Ran integration test.
Will validate robots.txt after deploy and check metrics later to see if bots are gone.

Reviewers: ojcm

Reviewed By: ojcm

Subscribers: #scieng

Differential Revision: https://phabricator.internal.justin.tv/D5774

twitchscience/spade_edge,twitchscience/spade_edge