doc: add string_decoder doc

doc/api/string_decoder.markdown

@@ -0,0 +1,24 @@
+# StringDecoder
+
+    Stability: 2 - Unstable
+
+To use this module, do `require('string_decoder')`. StringDecoder decodes a
+buffer to a string. It is a simple interface to `buffer.toString()` but provides
+additional support for utf8.
+
+    var StringDecoder = require('string_decoder').StringDecoder;
+    var decoder = new StringDecoder('utf8');
+
+    var cent = new Buffer([0xC2, 0xA2]);
+    console.log(decoder.write(cent));
+
+    var euro = new Buffer([0xE2, 0x82, 0xAC]);
+    console.log(decoder.write(euro));
+
+## Class: StringDecoder
+
+Accepts a single argument, `encoding` which defaults to `utf8`.
+
+### StringDecoder.write(buffer)
+
+Returns a decoded string.