Added placeholder doc for adding tests and bots

TBR=nednguyen@google.com

Review-Url: https://codereview.chromium.org/2940423002
Cr-Commit-Position: refs/heads/master@{#480263}

docs/speed/adding_tests_bots.md

+# Adding tests and bots
+
+## Adding performance tests
+
+See the
+[new benchmark policy](https://docs.google.com/document/d/1ni2MIeVnlH4bTj4yvEDMVNxgL73PqK_O9_NUm3NW3BA/edit)
+
+## Adding hardware
+
+This documentation is under construction. In the meantime, please reach out to
+benchmarking-dev@chromium.org.
\ No newline at end of file