[1] Improved links for Composer

- Linking to composer's git repo doesn't make any sense.
- There are no composer installation instructions on packagist.org, they
  are located on getcomposer.org/download
[1] Improved Composer tags + reflected composer changes

- "Due to the dev- prefix, this is known to be a dev version, and dev
  versions have "dev" stability. Because this constraint for a dev version
  is defined in the root package, it implicitly gets the @dev stability
  flag." https://igor.io/2013/02/07/composer-stability-flags.html
- Update should be used if a new package was added
- Update and install now always install dev packages, use --no-dev to only
  install production packages.
[1] Put filenames in code blocks and official Git notation
[2] Corrected list indention
[3] Added some inline code blocks
[3] Made code example more readable
[3] Reflected documentation split
[4] Emphasized standalone
[4] Added inline code blocks
[4] Added line breaks for better readability
[4] Made examples more readable
[5] Added lots of inline code blocks
[6] Added lots of inline code blocks
[7..23] Minor fixes
Merge pull request #1 from WouterJ/GrahamCampbell-docs

Doc fixes