Updated readme for district and subDistrict

Author: billinghamj

README.md

@@ -1,11 +1,11 @@
-[![Build Status](https://travis-ci.org/cblanc/postcode.js.png)](https://travis-ci.org/cblanc/postcode.js) 
+[![Build Status](https://travis-ci.org/cblanc/postcode.js.png)](https://travis-ci.org/cblanc/postcode.js)
 [![Dependency Status](https://gemnasium.com/cblanc/postcode.js.png)](https://gemnasium.com/cblanc/postcode.js)
 
 # Postcodes
 
 Utility methods for UK Postcodes.
 
-Included is a test suite that tests against all postcodes listed in the Ordnance Survey's postcode dataset as of January 2014. 
+Included is a test suite that tests against all postcodes listed in the Ordnance Survey's postcode dataset as of January 2014.
 
 ## Getting Started
 
@@ -16,25 +16,22 @@ Create an instance of Postcode to perform utility methods, like so
 ```javascript
 var Postcode = require("postcode");
 
-var postcode = new Postcode("po167gz");
+var postcode = new Postcode("ec1v9lb");
 ```
 
 Perform simple validations, parsing and normalisation
 
 ```javascript
-postcode.valid()      // => True
-
-postcode.normalise()  // => "PO16 7GZ"
-
-postcode.outcode()    // => "PO16"
-
-postcode.incode()     // => "7GZ"
-
-postcode.area()				// => "PO"
-
-postcode.sector()			// => "PO16 7"
-
-postcode.unit()				// => "GZ"
+postcode.valid()        // => true
+postcode.normalise()    // => "EC1V 9LB"
+
+postcode.outcode()      // => "EC1V"
+postcode.incode()       // => "9LB"
+postcode.area()         // => "EC"
+postcode.district()     // => "EC1"
+postcode.subDistrict()  // => "EC1V"
+postcode.sector()       // => "EC1V 9"
+postcode.unit()         // => "LB"
 ```
 
 Misc. Class Methods include
@@ -45,27 +42,47 @@ Postcode.validOutcode(outcode)
 
 ## Definitions
 
-### Outcode (#outcode)
+### Outcode
 
 The outward code is the part of the postcode before the single space in the middle. It is between two and four characters long. A few outward codes are non-geographic, not divulging where mail is to be sent. Examples of outward codes include "L1", "W1A", "RH1", "RH10" or "SE1P".
 
-### Inward Code (#inward)
+### Incode
 
 The inward part is the part of the postcode after the single space in the middle. It is three characters long. The inward code assists in the delivery of post within a postal district. Examples of inward codes include "0NY", "7GZ", "7HF", or "8JQ".
 
-### Postcode Area (#area)
+### Area
 
 The postcode area is part of the outward code. The postcode area is between one and two characters long and is all letters. Examples of postcode areas include "L" for Liverpool, "RH" for Redhill and "EH" Edinburgh. A postal area may cover a wide area, for example "RH" covers north Sussex, (which has little to do with Redhill historically apart from the railway links), and "BT" (Belfast) covers the whole of Northern Ireland.
 
-### Postcode Sector (#sector)
+### District
+
+The district code is part of the outward code. It is between two and four characters long. It does not include the trailing letter found in some outcodes. Examples of district codes include "L1", "W1", "RH1", "RH10" or "SE1".
+
+### Sub-District
+
+The sub-district code is part of the outward code. It is often not present, only existing in particularly high density London districts. It is between three and four characters long. It does include the trailing letter omitted from the district. Examples of sub-district codes include "W1A", "EC1A", "NW1W", "E1W" or "SE1P".
+
+Note: for outcodes not ending with a letter, `subDistrict` will return `null`. For example:
+
+```js
+new Postcode("SW1A 1AA").subDistrict()  // => SW1A
+new Postcode("E1W 1LD").subDistrict()   // => E1W
+new Postcode("PO16 7GZ").subDistrict()  // => null
+new Postcode("B5 5NY").subDistrict()    // => null
+```
+
+### Sector
 
 The postcode sector is made up of the postcode district, the single space, and the first character of the inward code. It is between four and six characters long (including the single space). Examples of postcode sectors include "SW1W 0", "PO16 7", "GU16 7", or "L1 8", "CV1 4".
 
-### Postcode Unit (#unit)
+### Unit
 
 The postcode unit is two characters added to the end of the postcode sector. Each postcode unit generally represents a street, part of a street, a single address, a group of properties, a single property, a sub-section of the property, an individual organisation or (for instance Driver and Vehicle Licensing Agency) a subsection of the organisation. The level of discrimination is often based on the amount of mail received by the premises or business. Examples of postcode units include "SW1W 0NY", "PO16 7GZ", "GU16 7HF", or "L1 8JQ".
 
-Source: [Wikipedia](http://en.wikipedia.org/wiki/Postcodes_in_the_United_Kingdom#Formatting)
+Sources:
+
+- https://en.wikipedia.org/wiki/Postcodes_in_the_United_Kingdom#Formatting
+- https://en.wikipedia.org/wiki/London_postal_district#Numbered_divisions
 
 ## Testing
 
@@ -81,4 +98,4 @@ A complete list of Postcodes can be obtained from the ONS Postcode Directory, wh
 
 MIT
 
-Contains Ordnance Survey Data © Crown Copyright & Database Right 2014
+Contains Ordnance Survey Data © Crown Copyright & Database Right 2014