docs: Make goals and non-goals clearer

Make httpstan's goals and non-goals clearer.

doc/contributing.rst

@@ -7,23 +7,36 @@ call functions in the Stan C++ library's ``stan::services`` namespace.
 **httpstan** was originally developed as a "backend" for a Stan interface
 written in Python, PyStan_.
 
-httpstan emerged out of a rewrite of an existing piece of software (PyStan)
-which was difficult to maintain. httpstan aims to be maintainable. Code
-contributions should be readable and easy to understand.
+Goals:
+
+- Provide access to frequently-used functions in Stan C++ library's ``stan::services`` namespace.
+- Minimize toil. Maintaining httpstan should require as little time as possible.
+
+Non-goals:
+
+- Provide access to functions other than those in the ``stan::services`` namespace.
+  There are only three exceptions to this non-goal: ``model_base_crtp.log_prob``,
+  ``model_base_crtp.write_array``, and ``stan::model::log_prob_grad``.
+
+If these goals and non-goals strike you as restrictive, we kindly remind you
+that httpstan is open source software which you are free to fork and customize.
 
 .. _shim: https://en.wikipedia.org/wiki/Shim_%28computing%29
 .. _PyStan: http://mc-stan.org/interfaces/pystan.html
 
 How to Make a Code Contribution
 ===============================
 
+Code contributions must be readable and easy to understand.
+httpstan emerged out of a rewrite of an existing piece of software (PyStan)
+which was difficult to maintain. httpstan aims to be maintainable.
+
 See `Contributing to PyStan`_ in the PyStan repository. Contributions
 to httpstan follow the same guidelines.
 
-There is one important difference between httpstan and pystan.
-httpstan provides an HTTP-based REST API.
+There is one important difference between httpstan and PyStan.
+httpstan provides access to Stan functions via an HTTP-based REST API.
 This REST API follows the conventions described in the document `API Design Guide
-<https://cloud.google.com/apis/design/>`_. Contributors interested in changing
-the behavior of the REST API should consult this document.
+<https://cloud.google.com/apis/design/>`_.
 
 .. _Contributing to PyStan: https://pystan-next.readthedocs.io/en/latest/contributing.html