Add some text to the README

Author: asmeurer

README.md

@@ -1,2 +1,32 @@
-# numpy-array-api-compat
-Compatibility layer for NumPy to support the Array API
+# NumPy Array API compatibility library
+
+This is a small wrapper around NumPy that is compatible with the [Array API
+standard](https://data-apis.org/array-api/latest/). See also [NEP 47](https://numpy.org/neps/nep-0047-array-api-standard.html).
+
+Unlike `numpy.array_api`, this is not a strict minimal implementation of the
+Array API, but rather just an extension of the main NumPy namespace with
+changes needed to be compliant with the Array API. See
+https://numpy.org/doc/stable/reference/array_api.html for a full list of
+changes. In particular, unlike `numpy.array_api`, this package does not use a
+separate Array object, but rather just uses `numpy.ndarray` directly.
+
+Note that some of the functionality in this library is backwards incompatible
+with NumPy.
+
+Library authors using the Array API may wish to test against `numpy.array_api`
+to ensure they are not using functionality outside of the standard, but prefer
+this implementation for end users who use NumPy arrays.
+
+## Usage
+
+To use this library replace
+
+```py
+import numpy as np
+```
+
+with
+
+```py
+import numpy_array_api_compat as np
+```