Update README.md

README.md

 Chisel IP-XACT packaging for Xilinx Vivado Design Suite
 =======================================================
 
-Helper scripts and Scala classes to simplify the generation of IP-XACT IP cores from Chisel. Uses Xilinx Vivado Design Suite to infer most of the interfaces (may require some massaging to fit your needs).
+Helper scripts and Scala classes to simplify the generation of IP-XACT IP cores from Chisel.
+Uses Xilinx Vivado Design Suite to infer most of the interfaces (may require some massaging
+to fit your needs).
 
+Basic Usage
+-----------
+Class `ModuleBuilder` provides an abstract base class for an executable object with a `main`
+method. You'll only need to provide a list of Chisel cores you'd like to wrap into IP-XACT.
+A full example can be found in `examples`; to run it, copy it to the project main directory
+and simply run `sbt run`. This will produce a new subdirectory `ip`, which contains further
+subdirectories for each core. You can use this directory as a repository base in Xilinx
+Vivado Design Suite, the cores can then be used in Vivado projects and IP integrator.
+
+Usage in Chisel Projects (*sbt*)
+--------------------------------
+The easiest way to setup `chisel-packaging` for your Chisel project w/`sbt` project is using
+**git subtree** (see [1] for an excellent intro to subtrees) as follows:
+
+  1. In your the base directory of your project, add a new remote *chisel-packaging*:
+     ```
+     git remote add chisel-packaging https://bitbucket.org/jkorinth/chisel-packaging.git
+	 ```
+  2. Setup a subtree in `packaging`:
+     ```
+	 git subtree add --prefix packaging chisel-packaging master --squash
+	 ```
+	 This will clone the `chisel-packaging` into the `packaging` folder.
+  3. Add a project dependency in your `build.sbt`, add lines:
+     ```
+lazy val packaging = project.in(file("packaging"))
+
+lazy val root = (project in file(".")).dependsOn(packaging)
+
+	 ```
+	 *Note that the empty lines are **not optional** - a quirk of sbt.*
+
+Now you're ready to subclass `chisel.packaging.ModuleBuilder` in your own code. Build the
+IP-XACT cores with `sbt run`.
+
+Gotchas
+-------
+  * The python script requires `vivado` to be in `PATH`, so you need to source the Vivado
+    settings scripts first.
+  * The repo is setup for Chisel 3.x; it can be used for Chisel 2.x, but that requires
+    manual changes - open an issue in the issue tracker if you need it.
+  * If you're using the `sbt` approach outlined above, make sure your `build.sbt` contains
+    the empty lines; they are required by `sbt`.
+
+[1]: https://www.atlassian.com/blog/git/alternatives-to-git-submodule-git-subtree
\ No newline at end of file