Watcher official install-guide

Currently watcher project does not have an official
install guide at [1]

This patch adds watcher install guide for rdo and debian.

install-guide is written following the document [2].

[1] https://docs.openstack.org/project-install-guide/ocata/
[2] https://docs.openstack.org/contributor-guide/project-install-guide.html

Change-Id: Idfae7286003f81222dadf91ddcaf95a42c7eb07f

install-guide/source/next-steps.rst

@@ -0,0 +1,9 @@
+.. _next-steps:
+
+Next steps
+~~~~~~~~~~
+
+Your OpenStack environment now includes the watcher service.
+
+To add additional services, see
+https://docs.openstack.org/project-install-guide/ocata/.