Add FAQ about adding custom tiles.

Author: HexDecimal

docs/faq.rst

@@ -9,6 +9,7 @@ This can either be your own custom tool or you can copy the Clock class from the
 `framerate.py <https://github.com/libtcod/python-tcod/blob/develop/examples/framerate.py>`_
 example.
 
+
 I get ``No module named 'tcod'`` when I try to ``import tcod`` in PyCharm.
 --------------------------------------------------------------------------
 
@@ -29,4 +30,23 @@ Once this file is saved to your projects root directory then PyCharm will detect
 Alternatively you can open the `Terminal` tab in PyCharm and run ``pip install tcod`` there.  This will install `tcod` to the currently open project.
 
 
+How do I add custom tiles?
+--------------------------
+
+Libtcod uses Unicode to identify tiles.
+To prevent conflicts with real glyphs you should decide on codepoints from a `Private Use Area <https://en.wikipedia.org/wiki/Private_Use_Areas>`_ before continuing.
+If you're unsure, then use the codepoints from ``0x100000`` to ``0x10FFFD`` for your custom tiles.
+
+Normally you load a font with :func:`tcod.tileset.load_tilesheet` which will return a :any:`Tileset` that gets passed to :func:`tcod.context.new`'s `tileset` parameter.
+:func:`tcod.tileset.load_tilesheet` assigns the codepoints from `charmap` to the tilesheet in row-major order.
+
+There are two ways to extend a tileset like the above:
+
+- Increase the tilesheet size vertically and update the `rows` parameter in :func:`tcod.tileset.load_tilesheet` to match the new image size, then modify the `charmap` parameter to map the new tiles to codepoints.
+  If you edited a CP437 tileset this way then you'd add your new codepoints to the end of :any:`tcod.tileset.CHARMAP_CP437` before using the result as the `charmap` parameter.
+  You can also use :any:`Tileset.remap` if you want to reassign tiles based on their position rather than editing `charmap`.
+- Or do not modify the original tilesheet.
+  Load the tileset normally, then add new tiles with :any:`Tileset.set_tile` with manually loaded images.
+
+
 .. _PyCharm: https://www.jetbrains.com/pycharm/

tcod/tileset.py

@@ -278,9 +278,8 @@ def load_tilesheet(path: Union[str, Path], columns: int, rows: int, charmap: Opt
     `columns` and `rows` is the shape of the tileset.  Tiles are assumed to
     take up the entire space of the image.
 
-    `charmap` is the character mapping to use.  This is a list or generator
-    of codepoints which map the tiles like this:
-    ``charmap[tile_index] = codepoint``.
+    `charmap` is a sequence of codepoints to map the tilesheet to in row-major order.
+    This is a list or generator of codepoints which map the tiles like this: ``charmap[tile_index] = codepoint``.
     For common tilesets `charmap` should be :any:`tcod.tileset.CHARMAP_CP437`.
     Generators will be sliced so :any:`itertools.count` can be used which will
     give all tiles the same codepoint as their index, but this will not map