Add details to README file and correct mistakes

- Add information about first past the post system
- Improve explanation of other electoral systems
- Add an 'Aim' section
- Correct mistakes and make small improvements to existing sections
- Reduce details in the 'Voters' section
Improve 'Coalitions' section (README file)

Move the section below the 'Elections' section and rephrase parts of it.
Merge branch 'master' of https://github.com/frunchy/ElectionSimulator