Update CONTRIBUTING document.

Add a quick note about design docs and try bot access.

Change-Id: I6a09dfd70b9e408fe59f9a39639a0b7b5965e4b4
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/70352
Auto-Submit: dsinclair <dsinclair@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 4830470..265e1ab 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -24,6 +24,9 @@
    place for long histories, discussion and debate, attaching screenshots, and
    linking to other associated bugs. Bugs are unnecessary for changes isolated
    enough to not need any of these.
+ * If the work being implemented is especially complex or large a design
+   document may be warranted. The document should be linked to the filled bug
+   and be set to publicly viewable.
  * If there isn't a bug and there should be one, please file a new bug.
  * Just because there is a bug in the bug system doesn't necessarily mean that
    a patch will be accepted.
@@ -95,6 +98,13 @@
 against the structure being received having the required version number in
 order to validate the new additions are available.
 
+## Trybot Access
+Changes must pass the try bots before the are merged into the repo. For your
+first few CLs the try bots will need to be triggered by a committer. After
+you've submitted 2-3 CLs you can request try bot access by emailing one of the
+OWNERS and requesting try bot access. This will allow you to trigger the bots
+on your own changes without needing a committer.
+
 ## Committers
 All changes committed to PDFium must be reviewed by a committer. Committers
 have done significant work in the PDFium code base and have a good overall