Briefly documented alternative input methods (IM character map files) in "Extending..."

docs/html/EXTENDING.html

@@ -740,5 +740,63 @@ effect.</p>
 
 </blockquote>
 
+<h2>Alternative Input Methods</h2>
+<blockquote>
+  <p>As of version 0.9.17, Tux&nbsp;Paint's "Text" tool can provide
+  alternative input methods for some languages.  For example, when
+  Tux&nbsp;Paint is running with a Japanese locale, the
+  right&nbsp;<b>[Alt]</b> can be pressed to change between Latin,
+  Romanized Hiragana and Katakana modes.  This allows native characters to
+  be entered into the "Text" tool by typing one or more keys on a keyboard
+  with Latin characters (e.g., a US QWERTY keyboard).</p>
+
+  <p>To create an input method for a new locale, create a text file
+  with a name based on the locale (e.g., "<code>ja</code>" for Japanese),
+  with "<code>.im</code>" as the extension (e.g., "<code>ja.im</code>").</p>
+
+  <p>The "<code>.im</code>" file can have multiple character mapping sections
+  for different character mapping modes.  For example, on a Japanese typing
+  system, typing <b>[K]</b>&nbsp;<b>[A]</b> in Hiragana mode generates a
+  different Unicode character than typing
+  <b>[K]</b>&nbsp;<b>[A]</b> in Katakana mode.</p>
+
+  <p>Start each character mapping section with the word "<code>section</code>",
+  the follow it with the mappings, one per line.  Each line should
+  contain (separated by whitespace):</p>
+  
+  <ul>
+  <li>the Unicode value of the character, in hexadecimal
+  <li>the keycode sequence (the ASCII characters that must be entered to
+    generate the Unicode character)
+  <li>a flag (or "<code>-</code>")
+  </ul>
+
+  <p>Example:</p>
+
+  <blockquote>
+  <p><code># Hiragana<br>
+  section<br>
+  304B &nbsp; ka &nbsp; -<br>
+  304C &nbsp; ga &nbsp; -<br>
+  304D &nbsp; ki &nbsp; -<br>
+  304E &nbsp; gi &nbsp; -<br>
+  &nbsp;<br>
+  # Katakana<br>
+  section<br>
+  30AB &nbsp; ka &nbsp; -<br>
+  30AC &nbsp; ga &nbsp; -<br>
+  30AD &nbsp; ki &nbsp; -<br>
+  30AE &nbsp; gi &nbsp; -
+  </code></p>
+  </blockquote>
+
+  <p><i>Note:</i> Blank lines will be ignored, as will any text following
+  a "<code>#</code>" (pound/hash) character &mdash; they can be used to
+  denote comments, as seen in the example above.</p>
+
+  <p><i>Note:</i> Flags need to be explained here.</p>
+
+</blockquote>
+
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