Add a README for the apidoc repository (#1324)

Signed-off-by: Uli Schlachter <psychon@znc.in>

build-utils/travis-apidoc.sh

@@ -61,6 +61,20 @@ diff -Nur . ../doc -I "Last updated" -I "<strong>Release</strong>:" \
   -I "<h2>API documentation for awesome, a highly configurable X window manager (version .*)\.</h2>" \
   -x .git | patch -p1
 
+# Create the readme
+cat > README.md <<END
+# Awesome API documentation
+
+This repository contains the built API documentation for the
+[awesome](https://github.com/awesomeWM/awesome) window manager. It is
+automatically updated via Travis when the master branch changes. Hence:
+
+## Do NOT send pull requests here
+
+Instead, please update the source code of
+[awesome](https://github.com/awesomeWM/awesome) instead.
+END
+
 git add --all .
 if git diff --cached --exit-code --quiet; then
   echo "Documentation has not changed."