update contribution guidelines (Xilinx#807)

* update contribution guidelines

CONTRIBUTING.md

@@ -1 +1,59 @@
 We welcome contributions to PYNQ.  Please first sign our <a href="https://www.clahub.com/agreements/Xilinx/PYNQ"> Contributor License Agreement</a>.
+
+If you have an idea how to improve PYNQ:
+
+1. Share your proposal via <a href="https://github.com/Xilinx/PYNQ/issues" target="_blank">Github issues</a>.
+
+   A general set of rules on what to submit:
+
+	 1. We welcome submissions to the pynq Python package, sdbuild flows and documentation.
+
+	 2. For development boards, we only host boards that we officially support. To 
+	 build new SDCard images for custom PYNQ enabled boards, we encourage users 
+	 to build a new board-only repository.  For reference, please see the 
+	 <a href="https://github.com/Avnet/Ultra96-PYNQ" target="_blank">Avnet Ultra96</a>
+	 or <a href="https://github.com/Xilinx/ZCU111-PYNQ" target="_blank">Xilinx ZCU111</a> 
+	 repositories. 
+     
+   3. For custom overlays, we only host overlays that we officially support. To 
+   create new custom overlays, we encourage users to build a new overlay 
+   repository. For reference, please see the 
+   <a href="https://github.com/Xilinx/PYNQ-HelloWorld" target="_blank">PYNQ-HelloWorld</a> 
+   and <a href="https://github.com/Xilinx/BNN-PYNQ" target="_blank">BNN-PYNQ</a> repositories.
+
+   4. For Microblaze peripheral drivers, we encourage users to submit their C/C++ within 
+   a Jupyter notebook using the microblaze IPython magic.  Please see 
+   the <a href="https://github.com/Xilinx/PYNQ/tree/master/boards/Pynq-Z1/base/notebooks/microblaze" target="_blank">Microblaze example notebooks</a> 
+   for how to write custom device drivers.
+
+
+2. Submitting your pull request:
+
+	1. Fork this repository to your own github account using the *fork* button above.
+
+	2. Clone the fork to a local computer using *git clone*. Checkout the branch you want to work on.
+
+	3. You can modify the Vivado projects, Python source code, or notebooks.
+
+	4. Modify the documentation if necessary.
+
+	5. Make sure your patch follows code standards:
+		1. <a href="https://www.doc.ic.ac.uk/lab/cplus/cstyle.html" target="_blank">C/C++ code</a>
+		2. <a href="https://www.python.org/dev/peps/pep-0008/" target="_blank">Python / Jupyter notebook</a>
+
+	6. Use *git add*, *git commit*, *git push* to add changes to your fork.
+
+	7. Submit a pull request by clicking the *pull request* button on your github repo:
+		1. The <a href="https://github.com/Xilinx/PYNQ" target="_blank">master branch</a> should always be
+		   treated as stable and clean. Only hot fixes are allowed to be pull-requested. The hot fix is supposed
+           to be very important such that without this fix, a lot of things break.
+        2. For new features, small bug fixes, doc updates, and many other fixes, users should pull request against
+           the development branch, which has the newest image version. For example, if we have released 
+           image v2.3 but you see a new branch `image_v2.4`, then you should pull request against 
+           `image_v2.4` branch.
+
+	Check the <a href="http://git.huit.harvard.edu/guide/" target="_blank">guide to git</a> for more information.
+
+3. We will review your contribution and, if any additional fixes or modifications are 
+necessary, may provide feedback to guide you. When accepted, your pull request will 
+be merged to the repository.

README.md

@@ -41,21 +41,8 @@ In Linux, you can rebuild the overlay by running *make* in the corresponding ove
 
 ## Contribute
 
-Contributions to this repository are welcomed. To submit a project for inclusion:
-
-1. Fork this repository to your own github account using the *fork* button above.
-
-2. Clone (download) the fork to a local computer using *git clone*.
-
-3. You can modify the Vivado project, bitstream, SDK project, Python source code, or notebook in the corresponding folders.
-
-4. Modify the documentation if necessary.
-
-5. Use *git add*-->*git commit*-->*git push* to add changes to your fork.
-
-6. Then submit a pull request by clicking the *pull request* button on your github repo.
-
-Check the <a href="http://git.huit.harvard.edu/guide/" target="_blank">guide to git</a> for more information.
+Contributions to this repository are welcomed. Please refer to <a href="https://github.com/Xilinx/PYNQ/blob/master/CONTRIBUTING.md" target="_blank">CONTRIBUTING.md</a> 
+for how to improve PYNQ.
 
 ## Support