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
Log JavaScript errors in the console as well
Fix file upload styling
Merge pull request #2036 from DavidOliver/readme-2.4

Update README
If a token expires, remove it so future requests will work
Release 2.4RC1
Set default config values for new installations. Closes #2041
For new installations, turn on XSRF by default. For updates, disable it. This is purely for compatibility reasons. RE: #1874
Ensure resaving an Event doesn't reset it's Target. Fixes #2047
Add data-interactive attribute to plugin elements. No real effect just yet (future planning). RE: #2019
disabled multiviews
Merge pull request #2048 from jensscherbl/multiviews

Disable multiviews in .htaccess
Resolve full titles for the DSE/EE Attach to Pages Select Box
Merge branch 'integration' of github.com:symphonycms/symphony-2 into integration
Resolve full page titles for Datasource/Events Index
Change default prepopulation to now instead of today for consistency. Fixes #2053
Remove XSRF from Frontend, add a token to the params instead. RE: #1874, RE: #2046
Restore collapsible states

Fixes #2030
Add [data-interactive] to plugin initialisation

Closes #2019