Add documentation detailing Drupal's support life-cycle

Barring a short note couched within "Which version of Drupal core should I run?", there is no concrete information on Drupal's support policy. Yes, users can go to the Drupal project page and view the supported releases. However, this does not provide reliable information on how long or until when support for a release will last or for that matter why the current policy is being followed.

Perhaps something like this will work as a good starting point:

Drupal core's support life-cycle

Drupal core follows a strict support life-cycle for its releases. This enables administrators to better manage their sites, developers, their code and vendors, their own support policies.

At any point in time, only the two most recent releases of Drupal core are supported. In other words, when Drupal 7 was released, support for Drupal 5 ended and the two supported releases became Drupal 6 and Drupal 7. Similarly, when Drupal 8 is released, support for Drupal 6 will end and the two supported releases will then be Drupal 7 and Drupal 8 as illustrated in the table below:

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[There will possibly need to be some mention of why the cycle only spans 2 releases. Reasons probably include the availability of resources, the support cycles of dependencies such as PHP and so on.]

I expect that stuff like this need a certain degree of review prior to publishing it; hence this issue rather than a book page.

-K