Prerender: Update README.md and WATCHLISTS

This CL...

- adds "Trigger" in the terminology section in README.md.
- adds how to watch relevant code changes in README.md.
- removes the no_state_prefetch directories from the WATCHLISTS entry
  for prerendering. The new prerendering implementation deviates from
  the NoStatePrefetch implementation. Probably watchers are no longer
  interested in changes for them.

Bug: 1126305
Change-Id: Ia07a60f1ae0c6f26de54c4cfa75dadf64386b25c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636979
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844997}

WATCHLISTS

@@ -1595,9 +1595,7 @@
       'filepath': 'components/search_engines/prepopulated_engines.json',
     },
     'prerender': {
-      'filepath': 'chrome/browser/prefetch/no_state_prefetch/' \
-                  '|content/browser/prerender/' \
-                  '|components/no_state_prefetch/'
+      'filepath': 'content/browser/prerender/'
     },
     'presentation': {
       'filepath': 'content/browser/presentation/' \

content/browser/prerender/README.md

@@ -2,6 +2,9 @@ This directory contains the experimental Prerender2 implementation
 (https://crbug.com/1126305). This feature is now behind the
 `blink::features::kPrerender2` flag.
 
+If you're interested in relevant code changes, join the
+prerendering-reviews@chromium.org group.
+
 # Summary
 
 Prerendering is "pre"-rendering, it's about pre-loading and rendering a page
@@ -12,6 +15,8 @@ The Prerender2 is the new implementation of prerendering.
 
 # Terminology
 
+- **Trigger**: "Trigger" is an entry point to start prerendering. Currently,
+  `<link rel=prerender>` is the only trigger.
 - **Activate**: The Prerender2 runs navigation code twice: navigation for
   prerendering a page, and navigation for displaying the prerendered page.
   "Activate" indicates the latter navigation.