Write documentation to help users write their first few algorithms

[NickDraper]

This will need to explain the algorithm lifecycle, and describe all of the object provided within an algorithm and how to use them.

We need to aim this at people with minimal programming knowledge and include plenty of examples.

It should be written assuming that people are using the installed version of Mantid and building using the Build.bat available from the UserAlgorithms directory.

[NickDraper]

This issue was originally trac ticket 174