docs: include Base64 in the filter API reference

docs/filter/base64.md

@@ -0,0 +1,13 @@
+# Base64
+
+```{eval-rst}
+.. automodule:: numcodecs.base64
+
+.. autoclass:: Base64
+
+    .. autoattribute:: codec_id
+    .. automethod:: encode
+    .. automethod:: decode
+    .. automethod:: get_config
+    .. automethod:: from_config
+```

docs/filter/index.md

@@ -11,4 +11,5 @@ packbits
 categorize
 astype
 shuffle
+base64
 ```

docs/release.md

@@ -14,6 +14,11 @@
 
 ## Unreleased
 
+### Documentation
+
+* Add the Base64 codec to the filter API reference, with an encoding and decoding example.
+  By {user}`Wentao Tong <tongwentao>`, {issue}`587`
+
 (release_0.17.1)=
 
 ## 0.17.1

src/numcodecs/base64.py

@@ -5,7 +5,16 @@
 
 
 class Base64(Codec):
-    """Codec providing base64 compression via the Python standard library."""
+    """Codec providing Base64 encoding via the Python standard library.
+
+    Examples
+    --------
+    >>> codec = Base64()
+    >>> codec.encode(b'hello')
+    b'aGVsbG8='
+    >>> codec.decode(b'aGVsbG8=')
+    b'hello'
+    """
 
     codec_id = "base64"