Add README docs about using unstable position data

We want to add an interface for interacting with unstable position data
to viewer-sinatra. This interface is an extension to the
`EveryPolitician::Legislature` class, which operates on the
`unstable/positions.csv` file in a legislature's directory in
everypolitician-data.

README.md

@@ -110,6 +110,31 @@ bundle exec rake test:all
 bundle exec rake
 ```
 
+## Internal documentation
+
+### Interacting with unstable data
+
+We use data from the `unstable/positions.csv` file in a legislature's directory to display cabinet memberships on the website. Because this data is unstable there currently aren't methods in [the `everypolitician` gem](https://github.com/everypolitician/everypolitician-ruby) for interacting with it, so viewer-sinatra has some of its own classes for dealing with it. These classes live in the `lib/everypolitician_extensions.rb` file and are documented below.
+
+#### Get all cabinet memberships for a person
+
+To print all of the known cabinet memberships for a person, you can do the following:
+
+```ruby
+house_of_commons = Everypolitician::Index.new.country('UK').legislature('Commons')
+person = house_of_commons.popolo.persons.find_by(name: 'Gordon Brown')
+
+person.cabinet_memberships.each do |membership|
+  puts "#{person.name} was #{membership.label} #{membership.start_date} - #{membership.end_date}"
+end
+```
+
+Which will output something like this:
+
+```
+Gordon Brown was Prime Minister of the United Kingdom 2007-06-27 - 2010-05-11
+Gordon Brown was Chancellor of the Exchequer 1997-05-02 - 2007-06-27
+```
 
 ## Sinatra, SASS, styling