doc: add punycode.js documentation

doc/api/punycode.markdown

@@ -0,0 +1,70 @@
+# punycode
+
+    Stability: 2 - Unstable
+
+[Punycode.js](http://mths.be/punycode) is bundled with Node.js v0.6.2+. Use
+`require('punycode')` to access it. (To use it with other Node.js versions,
+use npm to install the `punycode` module first.)
+
+## punycode.decode(string)
+
+Converts a Punycode string of ASCII code points to a string of Unicode code
+points.
+
+    // decode domain name parts
+    punycode.decode('maana-pta'); // 'mañana'
+    punycode.decode('--dqo34k'); // '☃-⌘'
+
+## punycode.encode(string)
+
+Converts a string of Unicode code points to a Punycode string of ASCII code
+points.
+
+    // encode domain name parts
+    punycode.encode('mañana'); // 'maana-pta'
+    punycode.encode('☃-⌘'); // '--dqo34k'
+
+## punycode.toUnicode(domain)
+
+Converts a Punycode string representing a domain name to Unicode. Only the
+Punycoded parts of the domain name will be converted, i.e. it doesn't matter if
+you call it on a string that has already been converted to Unicode.
+
+    // decode domain names
+    punycode.toUnicode('xn--maana-pta.com'); // 'mañana.com'
+    punycode.toUnicode('xn----dqo34k.com'); // '☃-⌘.com'
+
+## punycode.toASCII(domain)
+
+Converts a Unicode string representing a domain name to Punycode. Only the
+non-ASCII parts of the domain name will be converted, i.e. it doesn't matter if
+you call it with a domain that's already in ASCII.
+
+    // encode domain names
+    punycode.toASCII('mañana.com'); // 'xn--maana-pta.com'
+    punycode.toASCII('☃-⌘.com'); // 'xn----dqo34k.com'
+
+## punycode.ucs2
+
+### punycode.ucs2.decode(string)
+
+Creates an array containing the decimal code points of each Unicode character
+in the string. While [JavaScript uses UCS-2
+internally](http://mathiasbynens.be/notes/javascript-encoding), this function
+will convert a pair of surrogate halves (each of which UCS-2 exposes as
+separate characters) into a single code point, matching UTF-16.
+
+    punycode.ucs2.decode('abc'); // [97, 98, 99]
+    // surrogate pair for U+1D306 tetragram for centre:
+    punycode.ucs2.decode('\uD834\uDF06'); // [0x1D306]
+
+### punycode.ucs2.encode(codePoints)
+
+Creates a string based on an array of decimal code points.
+
+    punycode.ucs2.encode([97, 98, 99]); // 'abc'
+    punycode.ucs2.encode([0x1D306]); // '\uD834\uDF06'
+
+## punycode.version
+
+A string representing the current Punycode.js version number.