Move API documentation into docs directory, and convert to markdown. (#588)

Author: drinckes

API.txt

@@ -0,0 +1,62 @@
+# Open Location Code Reference API
+
+## Version History
+
+- Version 1.0.0 / 2014-10-27 / [Doug Rinckes](https://github.com/drinckes), Google / Philipp Bunge, Google
+  - Initial public release
+
+## Public methods
+
+The following public methods should be provided by any Open Location Code implementation, subject to minor changes caused by language conventions.
+
+Note that any method that returns an Open Location Code should return upper case characters.
+
+Methods that accept Open Location Codes as parameters should be case insensitive.
+
+### `isValid`
+
+The `isValid` method takes a single parameter, a string, and returns a boolean indicating whether the string is a valid Open Location Code sequence or not.
+
+To be valid, all characters must be from the Open Location Code character set with exactly one separator.
+There must be an even number of at most eight characters before the separator.
+(Zero characters before the separator is valid.)
+
+### `isShort`
+
+The `isShort` method takes a single parameter, a string, and returns a boolean indicating whether the string is a valid short Open Location Code or not.
+
+A short Open Location Code is a sequence created by removing an even number of characters from a full Open Location Code.
+The resulting code must still include the separator character.
+
+### `isFull`
+
+Determines if a code is a valid full Open Location Code.
+
+Not all possible combinations of Open Location Code characters decode to valid latitude and longitude values.
+This checks that a code is valid and also that the latitude and longitude values are legal.
+Full codes must include the separator character and it must be after eight characters.
+
+### `encode`
+
+Encode a location into an Open Location Code. 
+This takes a latitude and longitude and an optional length.
+If the length is not specified, a code with 10 digits and an additional separator character will be generated.
+
+### `decode`
+
+Decodes an Open Location Code into the location coordinates. This method takes a string.
+If the string is a valid full Open Location Code, it returns an object with the lower and upper latitude and longitude pairs, the center latitude and longitude, and the length of the original code.
+
+### `shorten`
+
+Passed a valid full Open Location Code and a latitude and longitude this removes as many digits as possible (up to a maximum of six) such that the resulting code is the closest matching code to the passed location.
+A safety factor may be included.
+
+If the code cannot be shortened, the original full code should be returned.
+
+Since the only really useful shortenings are removing the first four or six characters, this method may be replaced with methods such as `shortenBy4` or `shortenBy6`.
+
+### `recoverNearest`
+
+This method is passed a valid short Open Location Code and a latitude and longitude, and returns the nearest matching full Open Location Code to the specified location.
+