Include entry points as an optional feature (#297)

* Entry points as an optional feature when adoption

* Update copy

* Fix grammar

* Revert change

* Update copy

Co-authored-by: Athan <kgryte@gmail.com>

Author: honno

spec/conf.py

@@ -143,4 +143,5 @@
     ),
     "durole": ("http://docutils.sourceforge.net/docs/ref/rst/" "roles.html#%s", ""),
     "dudir": ("http://docutils.sourceforge.net/docs/ref/rst/" "directives.html#%s", ""),
+    "pypa": ("https://packaging.python.org/%s", ""),
 }

spec/purpose_and_scope.md

@@ -369,6 +369,31 @@ recommended not to add other functions or objects, because that may make it
 harder for users to write code that will work with multiple array libraries.
 
 
+### Discoverability
+
+To assist array-consuming libraries which need to create arrays originating from multiple conforming array implementations, conforming implementations may provide an {pypa}`entry point <specifications/entry-points/>` in order to make an array API namespace discoverable. For example,
+
+```python
+from importlib.metadata import entry_points
+
+try:
+    eps = entry_points()['array_api']
+    ep = next(ep for ep in eps if ep.name == 'package_name')
+except TypeError:
+    # The dict interface for entry_points() is deprecated in py3.10,
+    # supplanted by a new select interface.
+    ep = entry_points(group='array_api', name='package_name')
+
+xp = ep.load()
+```
+
+An entry point must have the following properties:
+
+-   **group**: equal to `array_api`.
+-   **name**: equal to the package name.
+-   **object reference**: equal to the array API namespace import path.
+
+
 * * *
 
 ## Conformance