CONTRIBUTING.md: New

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# Contributing to SageMath #
+
+SageMath is an open-source project and has a very welcoming developer
+community. We invite newcomers to help improve the code or contribute
+in other ways.
+
+Our mission is:
+
+    Creating a viable free open source alternative to
+    Magma, Maple, Mathematica and Matlab.
+
+SageMath can give students and other contributors a wonderful
+experience in developing in a large-scale, supportive, and rigorous
+environment.
+
+All code is peer-reviewed, all functions must be documented and
+unit-tested on a variety of platforms and systems. Python is used as a
+base language, Git is used as the source code revision system, and
+Trac is used to track all support/development issues. All of these
+tools are very capable and used in many other small and large-scale
+projects.  From a software engineering standpoint, SageMath is a
+wonderful project to be engaged in.
+
+The mailing lists are extremely supportive and responsive. There are
+several hundred people that have contributed code directly to SageMath
+over the years, including many leaders in mathematics, as well as
+software professionals, graduate students, and undergraduate students.
+
+Contributions of all sorts are heartily welcomed, but attention is
+paid to making sure that the system does not become bloated and
+disorganized. The mission statement allows for a very wide variety of
+contributions.
+
+See [FAQ: Contributing to Sage](https://doc.sagemath.org/html/en/faq/faq-contribute.html)
+for more information.