Add top-level documentation to echidna

@drjeannewilson pointed out during some discussions that, for a new user there is a lack of documentation explaining the basic code structure:

[x] What is in each of the top-level directories?
[x] What are the main objectives of each part of the code?
[x] What are the first steps for a new user to familiarise themselves with the code?
[x] Perhaps have some entry-level example scripts:
- How do you make a basic Spectra object?
- How do you fill a Spectra object with e.g. some random numbers?
- How do you smear a spectrum?
- How do you plot a spectrum?
- How do you create a very simple limit setting/fitting routine?

snoplusuk / echidna