Merge pull request picassodev#105 from streeve/contributing

Add contribution guide

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Contributing
+
+Contributing to Picasso is easy: just open a [pull
+request](https://help.github.com/articles/using-pull-requests/). Make
+`master` the destination branch on the [Picasso
+repository](https://github.com/PicassoDev/Picasso) and allow edits from
+maintainers in the pull request.
+
+Your pull request must pass Picasso's tests, which includes using the coding
+style from `.clang-format` (enforced with clang-format-14) and adding doxygen
+documentation, and be reviewed by at least one Picasso developer.
+
+`pre-commit` is a useful tool for ensuring feature branches are ready for
+review by running automatic checks locally before a commit is made.
+[Installation details](https://pre-commit.com/#install) (once per system) and
+[activation details](https://pre-commit.com/#usage) (once per repo) are
+available.