Adding howto file for diff.

Author: yuri

doc/howto.md

@@ -0,0 +1,26 @@
+
+A simple diff of two strings:
+
+    > require("diff")
+    > for _, token in ipairs(diff.diff("This is a test", "This was a test!")) do 
+    >> print(token[1], token[2])
+    >> end
+    This    same
+    is      out
+    was     in
+            same
+    a       same
+            same
+    test    out
+    test!   in
+            same
+
+That is, diff.diff(old, new) returns a table of pairs, each consisting of a string
+and it's status: "same" (the string is present in both), "in" (the string appeared
+in <i>new</i>, or "out" (the string was present in <i>old</i> but was removed).
+
+Alternatively, you can just generate an HTML for this diff:
+
+    > = diff.diff("This is a test", "This was a test!"):to_html()
+    This <del>is</del><ins>was</ins> a <del>test</del><ins>test!</ins>
+