Add travis test to check for any <image>/README.md changes

- the :'' is to ensure the yml parser doesn't think it is a mapped value

.travis.yml

@@ -10,7 +10,7 @@ install:
 script:
   - files="$(find -name '*.md' -print0 | xargs -0 markdownfmt -l)";
     if [ "$files" ]; then
-      echo >&2 "Need markdownfmt:";
+      echo >&2 'Need markdownfmt:';
       echo >&2 "$files";
       echo >&2;
       echo "$files" | xargs markdownfmt -d >&2;
@@ -28,3 +28,13 @@ script:
       echo >&2 "Too long (or too many lines):$failed";
       exit 1;
     fi
+  - if [ "$TRAVIS_PULL_REQUEST" != 'false' ]; then
+      if [ "$(git diff --numstat "$TRAVIS_COMMIT_RANGE" -- '*/README.md')" ]; then
+        echo >&2 'Error:'' at least one repo README.md has changed';
+        echo >&2 'These files are autogenerated, so it is unnecessary to modify them';
+        echo >&2 'Please update content.md and docker-library-bot will take care of README.md';
+        echo >&2 'See:'' https://github.com/docker-library/docs/#image-namereadmemd';
+        echo >&2;
+        exit 1;
+      fi
+    fi