README: improve config file docs

Bug: None
Test: read in gitiles
Change-Id: If931b94cfd2235f5a7b9e946c0da3e5099455149
diff --git a/README.md b/README.md
index d1ab954..6deec17 100644
--- a/README.md
+++ b/README.md
@@ -49,12 +49,15 @@
 
 ## PREUPLOAD.cfg
 
-This file are checked in the top of a specific git repository.  Stacking them
+This file is checked in the top of a specific git repository.  Stacking them
 in subdirectories (to try and override parent settings) is not supported.
 
 ## Example
 
 ```
+# Per-project `repo upload` hook settings.
+# https://android.googlesource.com/platform/tools/repohooks
+
 [Options]
 ignore_merged_commits = true
 
@@ -126,13 +129,20 @@
 that is executed. The key is used as the name of the hook for reporting purposes,
 so it should be at least somewhat descriptive.
 
+Whitespace in the key name is OK!
+
+The keys must be unique as duplicates will silently clobber earlier values.
+
+You do not need to send stderr to stdout.  The tooling will take care of
+merging them together for you automatically.
+
 ```
 [Hook Scripts]
-my_first_hook = program --gogog ${PREUPLOAD_FILES}
-another_hook = funtimes --i-need "some space" ${PREUPLOAD_FILES}
-some_fish = linter --ate-a-cat ${PREUPLOAD_FILES}
-some_cat = formatter --cat-commit ${PREUPLOAD_COMMIT}
-some_dog = tool --no-cat-in-commit-message ${PREUPLOAD_COMMIT_MESSAGE}
+my first hook = program --gogog ${PREUPLOAD_FILES}
+another hook = funtimes --i-need "some space" ${PREUPLOAD_FILES}
+some fish = linter --ate-a-cat ${PREUPLOAD_FILES}
+some cat = formatter --cat-commit ${PREUPLOAD_COMMIT}
+some dog = tool --no-cat-in-commit-message ${PREUPLOAD_COMMIT_MESSAGE}
 ```
 
 ## [Builtin Hooks]