ICU-20835 Update instructions for commit checker.

See #815

tools/commit-checker/README.md

@@ -24,7 +24,9 @@ Optional: save your Jira credentials in a `.env` file in this directory:
     JIRA_USERNAME=hello
     JIRA_PASSWORD=world
 
-This is required if you want to process sensitive tickets.
+This is required if you want to process sensitive tickets.  Note: JIRA_PASSWORD needs to be an API Token generated according to the following instructions:
+
+https://confluence.atlassian.com/cloud/api-tokens-938839638.html
 
 ## Usage
 
@@ -39,4 +41,31 @@ Run the tool and save the result into REPORT.md; set fixVersion to the upcoming
         --rev-range "release-63-1..upstream/maint/maint-64"
         > REPORT.md
 
-Create a branch and open a pull request so others can view the report easily.
+If the maintenance branch hasn't been cut yet, use upstream/master as the tip:
+
+        --rev-range "release-64-2..upstream/master"
+
+Note 1: These examples assume that your remote named "upstream" points to unicode-org/icu, the source of truth.
+
+Note 2: Please change the previous-release tag (release-63-1, release-64-2, etc) to the correct version at the time you run the tool!
+
+### Preview the Report
+
+To preview the report, render the Markdown file in your favorite Markdown preview tool, like grip:
+
+    $ pip3 install grip
+    $ grip REPORT.md
+     * Running on http://localhost:6419/ (Press CTRL+C to quit)
+ 
+### Sending for Review
+ 
+Before sending the report to ICU-TC, do some basic cleanup yourself by adjusting fix versions and resolutions on Jira issues.
+
+- Tickets with commits should be closed as Fixed, and fixed tickets without commits should be closed as Fixed by Other Ticket.  These tickets should have a fix version.
+- Tickets closed for any other reason, such as Duplicate, should not have a fix version.  Semantically, duplicate tickets inherit the fix version from the ticket to which they are duplicated.
+
+It should be possible to clear the first two sections of the report simply by correcting the ticket resolutions and fix versions in Jira.
+
+When ready, create a branch  and push to your fork so others can view the report easily.  Team members should close issues they own that are correctly fixed.  Re-generate the report periodically until it comes back clean.
+
+Note: REPORT.md is not intended to be merged back into master.