Various suggested README changes

I thought I'd tidy up the instructions on the file permissions in the readme, but one thing led to another...

[My version viewable here for checking](https://github.com/DavidOliver/symphony-2/blob/readme/README.markdown).

Suggested changes made in this commit:

- Table of contents added
- Link to Symphony website home page added in overview
- Webserver requirements made more accurate (I know Symphony works with Hiawatha and I think it does with Nginx, too?)
- Made the file permissions guidance clearer for those who may not be familiar with the concept
- Moved the install section above the updating section as this is more relevant to new users
- Moved some instructions about submodule locations which I believe are specific to "prior to 2.1" from the general update-via-Git instructions to the "prior to 2.1" section in order to simplify the update-via-Git section **(please check!)**
- Used the `git submodule update --init --recursive` command to fully initialise and update submodules recursively (should cover all eventualities)
- Used example.com for all example URLs
- General restructuring
- General wording/punctuation tweaks/corrections
- Used more typographically-oriented characters for apostrophes and speech marks (`‘`, `’`, `“`, `”`)
- Made title capitalisation consistent

Some people I imagine who in particular might want to check:

@allen, @brendo, @designermonkey, @michael-e, @nilshoerrmann