Update README

Covers using optional “bundle” extensions on install. (Update instructions still to consider and update!)

Various formatting and language tweaks.

Addition of extensions site link.

Permissions instructions recommend that only the root directory is made webserver-writeable, and the manifest and workspace directories will be created by the webserver user and thus automatically writeable by it.

Some Git commands made more explicit for Git beginners.

“Default workspace” re-termed to “example workspace”. If it was default, it would be in the main installation.
Correct chmod command in README
Merge pull request #2036 from DavidOliver/readme-2.4

Update README