adding Contributing.md (closes #413)

paging @kevinushey @jjallaire @thirdwing for comments

Contributing.md

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+
+## Contributing to Rcpp
+
+Interested in contributing? Great!  We really welcome bug reports and pull
+requests that expand and improve the functionality of Rcpp from all
+contributors. 
+
+### Reporting an Issue
+
+When reporting an issue, the most important thing you can provide is a
+[reproducible example](http://www.sscce.org/). Please include the smallest
+possible example that illustrates the issue -- when possible, provide a snippet
+of C++ code that can be run using `Rcpp::sourceCpp()`; if it's not possible or
+feasible to provide such an example, provide clear instructions on how to
+reproduce the problem. The less effort it takes to reproduce an issue, the more
+likely a contributor will be able to investigate the issue + resolve the
+problem. Also see this StackOverflow answer on
+[creating a reproducible example](http://stackoverflow.com/a/5963610/143305).
+
+Issues that cannot be reproduced are unlikely to receive attention, as it is
+often difficult, if not impossible, to ascertain whether Rcpp is truly the
+culprit, or what part of Rcpp could be responsible.
+
+Please also supply the output of `sessionInfo()` when reporting an issue.
+
+### Submitting a Pull Request
+
+Considering submitting a pull request? It is strongly recommended that you first
+post an issue outlining some motivation for your pull request to ensure that
+some discussion around the appropriate resolution for the issue occurs first.
+
+We would prefer it if your PR also included
+[unit tests](https://github.com/RcppCore/Rcpp/tree/master/inst/unitTests). Additions
+to the [ChangeLog](https://github.com/RcppCore/Rcpp/blob/master/ChangeLog) and
+[NEWS](https://github.com/RcppCore/Rcpp/blob/master/inst/NEWS.Rd) are also
+appreciated.
+
+Please note that C++ code in Rcpp must adhere to the C++98 standard. C++11 (and
+above) extensions are allowed, but must be appropriately guarded so that such
+code still successfully compiles under the C++98 standard.
+
+### Asking Questions
+
+Please direct general questions to the
+[Rcpp-devel](http://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel)
+mailing list (preferred, note that only subscribers can post), or alternatively
+post a question on
+[Stack Overflow](http://stackoverflow.com/questions/tagged/rcpp) using the
+`[Rcpp]` tag. 
+
+Are you a new user of Rcpp? You might find the
+[vignettes](https://cran.r-project.org/package=Rcpp) helpful in getting
+acquainted with the functionality Rcpp provides. The
+[Rcpp Gallery](http://gallery.rcpp.org/) contains a large number of examples.
+
+### Copyright
+
+Rcpp is released as a [CRAN](https://cran.r-project.org/package=Rcpp) package,
+licensed under the
+[GPL-2](http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html) (or later)
+[Open Source](https://opensource.org) /
+[Free Software](http://www.gnu.org/philosophy/free-sw.en.html)
+license -- just like R itself.  Aggregated works, i.e. code linking to R such as
+Rcpp will always be under this license.  Your contribution has to be under a
+compatible licence.  And to simplify matters, we strongly prefer contributions
+under the same terms of the GPL (>= 2) and will in general assume this license
+unless explicitly stated otherwise.