added TiffTags documentation

docs/reference/TiffTags.rst

@@ -0,0 +1,59 @@
+.. py:module:: PIL.TiffTags
+.. py:currentmodule:: PIL.TiffTags
+
+:py:mod:`TiffTags` Module
+=========================
+
+The :py:mod:`TiffTags` module exposes many of the stantard TIFF
+metadata tag numbers, names, and type information.
+
+.. method:: lookup(tag)
+
+    :param tag: Integer tag number
+    :returns: Taginfo namedtuple, From the ``TAGS_V2`` info if possible,
+        otherwise just populating the value and name from ``TAGS``.
+        If the tag is not recognized, "unknown" is returned for the name
+
+.. versionadded:: 3.1.0
+
+.. class:: TagInfo
+
+  .. method:: __init__(self, value=None, name="unknown", type=None, length=0, enum=None)
+
+     :param value: Integer Tag Number
+     :param name: Tag Name
+     :param type: Integer type from :py:attr:`PIL.TiffTags.TYPES`
+     :param length: Array length: 0 == variable, 1 == single value, n = fixed
+     :param enum: Dict of name:integer value options for an enumeration
+   
+  .. method:: cvt_enum(self, value)
+
+     :param value: The enumerated value name
+     :returns: The integer corresponding to the name. 
+
+.. versionadded:: 3.0.0
+
+.. py:attribute:: PIL.TiffTags.TAGS_V2
+
+    The ``TAGS_V2`` dictionary maps 16-bit integer tag numbers to
+    :py:class:`PIL.TagTypes.TagInfo` tuples for metadata fields defined in the TIFF
+    spec.
+
+.. versionadded:: 3.0.0
+
+.. py:attribute:: PIL.TiffTags.TAGS
+
+    The ``TAGS`` dictionary maps 16-bit integer TIFF tag number to
+    descriptive string names.  For instance:
+
+        >>> from PIL.TiffTags import TAGS
+        >>> TAGS[0x010e]
+        'ImageDescription'
+
+    This dictionary contains a superset of the tags in TAGS_V2, common
+    EXIF tags, and other well known metadata tags.
+
+.. py:attribute:: PIL.TiffTags.TYPES
+
+    The ``TYPES`` dictionary maps the TIFF type short integer to a
+    human readable type name.