Added rotate tiles tool

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+# Rotate tiles
+
+This tool can be used to transform an image containing a tileset into a new
+one with a single column of tiles, so can be loaded with ".incpng" and the
+result is easy to index.
+
+For example, given 6 tiles of 8x8 pixels in `origin.ong`:
+
+```
+012345
+```
+
+The command `./rotate.py origin.png 8 8 dest.png` will generate an image with
+the tiles rearranged like this:
+
+```
+0
+1
+2
+3
+4
+5
+```
+
+In this way tile 0 is in `addr`, tile 1 in `addr + 64`, tile 2 in `addr + 128`, etc.
+
+## Requirements
+
+The tool requires:
+
+- Python 3
+- PIL (or Pillow)
+
+For example, you can install Pillow with:
+
+```
+python3 -m pip install --upgrade pip
+python3 -m pip install --upgrade Pillow
+```
+