Add missing README.rst in elements

Currently dib-lint ensures all elements have README.rst[1], and this
casuses failure in pep8 job because some elements doesn't have
README.rst.
This patch adds missing README.rst to fix pep8 failures to unblock
gate.

[1] Ia5252ddd89b5ae5c6e9a12a66ef10f912fd54da5

Change-Id: I0602de2323aedd5d3334d9a06fe23a3ef5daf4b7
(cherry picked from commit 632ea261a2)

elements/instack-vm/README.rst

@@ -1,4 +1,5 @@
 Build an instack vm image
+==============
 
 This element allows building an instack vm image using diskimage-builder. To build
 the image simply include this element and the appropriate distro element.
@@ -6,4 +7,4 @@ For example:
 
 disk-image-create -a amd64 -o instack \
     --image-size 30 \
-    fedora instack-vm
+    fedora instack-vm

elements/pip-and-virtualenv-override/README.rst

@@ -1,3 +1,6 @@
+pip-and-virtualenv-override
+==============
+
 This element will override the behavior from the pip-and-virtualenv element
 from tripleo-image-elements so that python-pip and python-virtualenv are never
 installed.

elements/undercloud-install/README.rst

@@ -0,0 +1,4 @@
+undercloud-install
+==============
+
+Base element for undercloud installation