Add some design guidelines to docs

doc/index.rst

@@ -15,6 +15,39 @@ implementations for:
 :mod:`arraycontext` started life as an array abstraction for use with the
 :mod:`meshmode` unstrucuted discretization package.
 
+Design Guidelines
+-----------------
+
+Here are some of the guidelines we aim to follow in :mod:`arraycontext`. There
+exist numerous other, related efforts, such as the `Python array API standard
+<https://data-apis.org/array-api/latest/purpose_and_scope.html>`__. These
+points may aid in clarifying and differentiating our objectives.
+
+- The array context is about exposing the common subset of operations
+  available in immutable and mutable arrays. As a result, the interface
+  does *not* seek to support interfaces that provide, enable, or are typically
+  used only with in-place mutation.
+
+  For example: The equivalents of :func:`numpy.empty` were deprecated
+  and will eventually be removed.
+
+- Each array context offers a specific subset of of :mod:`numpy` under
+  :attr:`arraycontext.ArrayContext.np`. Functions under this namespace
+  must be unconditionally :mod:`numpy`-compatible, that is, they may not
+  offer an interface beyond what numpy offers. Functions that are
+  incompatible, for example by supporting tag metadata
+  (cf. :meth:`arraycontext.ArrayContext.einsum`) should live under the
+  :class:`~arraycontext.ArrayContext` directly.
+
+- Similarly, we strive to minimize redundancy between attributes of
+  :class:`~arraycontext.ArrayContext` and :attr:`arraycontext.ArrayContext.np`.
+
+  For example: ``ArrayContext.empty_like`` was deprecated.
+
+- Array containers are data structures that may contain arrays.
+  See :mod:`arraycontext.container`. We strive to support these, where sensible,
+  in :class:`~arraycontext.ArrayContext` and :attr:`arraycontext.ArrayContext.np`.
+
 Contents
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