Document how set traffic to the latest revision (#498)

Closes
#466

README.md

@@ -215,6 +215,13 @@ jobs:
       revision_traffic: 'my-revision=10' # percentage
     ```
 
+    To update traffic to the latest revision, use the special tag "LATEST":
+
+    ```yaml
+    with:
+      revision_traffic: 'LATEST=100'
+    ```
+
 -   `tag_traffic`: (Optional, mutually-exclusive with `revision_traffic`)
     Comma-separated list of tag traffic assignments.
 

action.yml

@@ -123,16 +123,22 @@ inputs:
 
   revision_traffic:
     description: |-
-      Comma separated list of traffic assignments in the form REVISION-NAME=PERCENTAGE.
-      REVISION-NAME must be the name for a revision for the service as returned
-      by 'gcloud beta run list revisions'. PERCENTAGE must be an integer
-      percentage between 0 and 100 inclusive.
-      Ex service-nw9hs=10,service-nw9hs=20 Up to 100 percent of traffic may be
+      Comma separated list of traffic assignments in the form
+      REVISION-NAME=PERCENTAGE. REVISION-NAME must be the name for a revision
+      for the service as returned by 'gcloud beta run list revisions'.
+      PERCENTAGE must be an integer percentage between 0 and 100 inclusive. Ex
+      service-nw9hs=10,service-nw9hs=20 Up to 100 percent of traffic may be
       assigned. If 100 percent of traffic is assigned, the Service traffic is
       updated as specified. If under 100 percent of traffic is assigned, the
       Service traffic is updated as specified for revisions with assignments and
       traffic is scaled up or down down proportionally as needed for revision
       that are currently serving traffic but that do not have new assignments.
+      For example assume revision-1 is serving 40 percent of traffic and
+      revision-2 is serving 60 percent. If revision-1 is assigned 45 percent of
+      traffic and no assignment is made for revision-2, the service is updated
+      with revsion-1 assigned 45 percent of traffic and revision-2 scaled down
+      to 55 percent. You can use "LATEST" as a special revision name to always
+      put the given percentage of traffic on the latest ready revision
     required: false
 
   tag_traffic: