Merge pull request QMCPACK#5678 from prckent/contributing_doc

Add CONTRIBUTING.md

Author: ye-luo

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+# Contributing to QMCPACK
+
+QMCPACK is fully open source, and contributions of any size are very welcome.
+
+Guidelines and examples of different ways to help are given in the QMCPACK manual
+https://qmcpack.readthedocs.io/en/develop/introduction.html#contributing-to-qmcpack . We aim to make contributing straightforward
+with lightweight policies typical of any mid-sized project. Contributions to the manual are described at
+https://qmcpack.readthedocs.io/en/develop/contributing.html -- even the smallest typo fixes are welcome. 
+
+Please open an Issue if you have any questions on features, roadmap, tutorials, installation, possible bugs etc. If you are already
+running QMCPACK, provide as much detail as possible in the report (e.g. copies of Nexus and QMCPACK input files.)
+
+If you like the project but don't have time to contribute, you can still support it by adding a star on GitHub or by citing QMCPACK
+when appropriate. 
+
+Thanks for your interest in improving QMCPACK.