Improve module information

Below is my attempt:

What is Read Only Mode?

Read Only Mode provides an alternate to the built in Maintenance Mode in Drupal. Instead of displaying a static text file to users while the site is in maintenance mode, Read Only Mode will allow access (reading) of new content while preventing the addition of new content (posting / submitting forms / etc). This allows the site to remain functional while maintenance is performed. This module also provides messaging to users and administrators to indicate that the site is in maintenance mode.

Quick set of features:

• Put the site in maintenance mode without taking your site offline
• Easy to configure whitelist of forms that should remain available
• Easy to configure custom warning messages
• Administrative permissions that override Read Only Mode access restrictions

Who Is this for?

This module is intended for site administrators and developers. The module provides a great way to lock down or freeze a production server so that maintenance or large deployments can occur without taking the site offline. In a typical example, Read Only Mode is activated on the production server, the database is copied to a development server where work is done, and then the database is pushed back to the production server and Read Only Mode disabled.

Usage and configuration

The module can be configured in the Site Maintenance settings page alongside the built in maintenance mode settings. Simply choose to enable the mode and your site is now locked down to content additions. Read Only Mode and Maintenance mode are separate and you do not need to enable Maintenance Mode to enable Read Only Mode.

Further configuration is possible to provide custom warnings and errors to users and to whitelist certain forms that your site deems 'safe' during Read Only Mode.
For example, a form may submit content through email or by accessing a callback on a web service and no data is stored in the Drupal database. It may make sense to allow site users to access these forms even when the site is in Read Only Mode.

In addition to a whitelist, the site also provides a permission that overrides access restrictions and can be given to site administrators.

Drush Support

Since Read Only Mode is defined by a single variable, it is easy to use Drush to enable and disable the mode.

Check the status:
drush vget readonlymode

Turn on Read Only Mode:
drush vset readonlymode 1

Turn off Read Only Mode:
drush vset readonlymode 0

Sorry, the drush commands were wrong.

The variable is site_readonly not readonlymode.

Can you update that for me?

Now that #1882296 has been backported, i've also backported these doc updates as appropriate to the 6,x branch.