Add INSTALL.md with generic build instructions

Signed-off-by: Gabor Igloi <gabor.igloi@citrix.com>

Author: gaborigloi

INSTALL.md

@@ -0,0 +1,12 @@
+There are two ways of building and installing the repository:
+
+* Installing as an OPAM package using the
+  [`opam-repo-dev`](https://github.com/xapi-project/opam-repo-dev) OPAM
+  repository, possibly in combination with the `opam` file in this repository.
+
+* Creating a Docker image with all the dependencies pre-installed using
+  [`xs-shipyard`](https://github.com/lindig/xs-shipyard), and then running
+  `make` to build the project, followed by `make install` to install it
+
+Please see the READMEs of the above repositories for detailed build
+instructions.