add contributing file

.Rbuildignore

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 ^\.travis\.yml$
 ISSUE_TEMPLATE.md
+CONTRIBUTING.md
+

CONTRIBUTING.md

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+# How to Contribute
+
+If you're reading this and thinking of contributing, first, thanks so much! All contributions big and small are welcome. This document is a set of guidelines (not firm rules) to help the process.
+
+Thanks!
+
+
+## Reporting a Bug or Requesting a Feature
+Please open an issue at the project repository's issue tracker on GitHub.  If there is already an open issue describing your problem/request, feel free to join the discussion inside the existing issue, but please do not open a new one.
+
+
+## Submitting Changes
+You can submit a pull request (PR) to the project repository on GitHub. Please try to keep PR's as "small" as possible (don't try to combine several large changes into one PR).
+
+For small changes, the log can be simple, e.g. "fixed a typo". For large commits, make sure to give a reasonably detailed description of the change(s) (this can easily be the line for the `ChangeLog`; see section below).
+
+If the project uses a continuous integration (CI) service, then the PR must pass CI before it will be merged.
+
+
+## Coding Conventions
+For both C and R code, please use the following conventions:
+
+  * Indent using two spaces.
+  * Use the [Allman](https://en.wikipedia.org/wiki/Indent_style#Allman_style) control statement style.  Do not use braces (but do indent) for one liners following a control statement.
+  * Always put spaces after commas and around operators. So for example, use `foo(a, b, c)` not `foo(a,b,c)`; and use `x += 1` not `x+=1`.
+  * If you create new functionality (new function, new arguments to an old function), add or expand a test in the `tests/` sub-tree.
+  * The `master` branch of this repository should always pass `R CME check --as-cran` with no NOTEs and no WARNINGs.
+  * Update the `ChangeLog` file in the root of this directory explainng what you did, and end the line with your initials or full name.  For example:
+  ```
+  Fixed a memory leak in src/foo.c. (ABC)
+  ```
+
+
+## Copyright
+This document is released as [CC0, public domain](https://creativecommons.org/choose/zero/). Feel free to re-use as much of this as you like with or without attribution if you find the template useful.