GitLab

Updates:
- Guides added for adding experimental attribute to a story and when to
do so.
- How to run perf tests locally is added to the document

Bug: none
Change-Id: Ie2c3f34906d185cd3992e4a614c49f43e4c3ad4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210632Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771156}