llvmbot monitor: Add "First Failing" and "Failing Since" columns

- The "First Failing" column contains a link to the first failed
  build, if the bot has been failing for several builds.

- The "Failing Since" column shows the time elapsed since that first
  failed build finished. This can be useful for detecting bots which
  fail for a long time.
  This column will be colored in red if that time is >24h.

Change-Id: I19878c922c65f8a9f5548ab8e3ea942a57c5a1f6
diff --git a/monitor/README.txt b/monitor/README.txt
index 06f0cec..4917e1e 100644
--- a/monitor/README.txt
+++ b/monitor/README.txt
@@ -104,12 +104,16 @@
     that have gotten disconnected. If this time is greater than 24 hours, it will be shown
     in red.
   * "Duration": The length of the last build.
-  * "Build": The build number of the last finished build, which itself will be a link
+  * "Latest": The build number of the last finished build, which itself will be a link
     to the results page for that build.
   * "Failing steps": The failed build steps, if it was a failed build.
   * "Build In Progress": This will be "Yes" if there is a build currently running or "No"
     if there is not. If we cannot determine this, it will be left blank and you should
     check the builder's status page instead.
+  * "1st Failing": The number of the first failed build, if the bot fails for several
+    builds.
+  * "Failing Since": The time since the first failed build finished. This is useful for
+    spotting bots that fail for a long time.
 
 Note: "finished" here refers to the build ending be that by success, cancellation or
 failure.