doc: git mv to .md

* doc: rename .markdown references in content
* doc: rename to .md in tools
* doc: rename to .md in CONTRIBUTING.md

PR-URL: https://github.com/nodejs/node/pull/4747
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: techjeffharris
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/string_decoder.md

@@ -0,0 +1,30 @@
+# StringDecoder
+
+    Stability: 2 - Stable
+
+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.
+
+```js
+const StringDecoder = require('string_decoder').StringDecoder;
+const decoder = new StringDecoder('utf8');
+
+const cent = new Buffer([0xC2, 0xA2]);
+console.log(decoder.write(cent));
+
+const euro = new Buffer([0xE2, 0x82, 0xAC]);
+console.log(decoder.write(euro));
+```
+
+## Class: StringDecoder
+
+Accepts a single argument, `encoding` which defaults to `'utf8'`.
+
+### decoder.end()
+
+Returns any trailing bytes that were left in the buffer.
+
+### decoder.write(buffer)
+
+Returns a decoded string.