Add documentation for contributing

CONTRIBUTING.md

@@ -1,6 +1,29 @@
 If you want to contribute to Aesara, have a look at the instructions here:
 https://aesara.readthedocs.io/en/latest/dev_start_guide.html
 
+## Contribution Expectations
+
+This "Contribution Expectations" section is adapted from [Open Source
+Archetypes](https://opentechstrategies.com/archetypes) and released under a
+[CC-BY-SA](https://creativecommons.org/licenses/by-sa/4.0/) license.
+
+The current focus of the Aesara project is rapid, high quality development of a
+hackable, pure-Python symbolic tensor library with strong support for graph
+rewrites and transpilation to multiple backends including C,
+[JAX](https://github.com/google/jax>), and
+[Numba](https://github.com/numba/numba).
+
+We welcome patches with bug fixes, and we’re happy to answer questions if you’ve
+already put in some effort to find the answer yourself. Please note, however,
+that we’re _unlikely_ to consider new feature contributions or design changes
+unless there’s a strong argument that they are fully in line with our stated
+goals. If you’re not sure, just ask.
+
+Our technical scope and project governance may open up later, of course,
+especially after we release version 1.0. For now, though, we would characterize
+this project as being a mix of the "Rocket Ship To Mars" and "Specialty Library"
+archetypes (see https://opentechstrategies.com/archetypes for details about RStM
+and other open source archetypes).
 
 ## Migrating PRs from the original Theano Repo
 Aesara is actively merging new changes. If you have a pull request on the original Theano repository and would like to move it here use the following commands in your local Aesara repository:

README.rst

@@ -116,6 +116,16 @@ The current development branch of |Project Name| can be installed from GitHub, a
     pip install git+https://github.com/aesara-devs/aesara
 
 
+Contributing
+============
+
+We welcome bug reports and fixes and improvements to the documentation.
+
+For more information on contributing, please see the
+`contributing guide <https://github.com/aesara-devs/aesara/CONTRIBUTING.md>`.
+
+A good place to start contributing is by looking through the issues
+`here <https://github.com/aesara-devs/aesara/issues`.
 
 Support
 =======