Document the JRE version used with JIRA

Use case:

A customer asked me which version of Java to use with JIRA 6.4.4. I said the only way to know what Atlassian was using was to run the Linux or Windows installer. They asked me to find out without running the installer (https://answers.atlassian.com/questions/15833150/how-to-find-the-jre-version-in-the-jira-installer) but I was not able to.

The documentation currently just says use a version >= Java 1.8.0_40
But the exact version that is shipped with the JIRA installer is not documented anywhere

Suggestion: add the JVM version README,txt file that is already shipped with the JIRA server tar.gz packages. It looks like the JIRA version is already added there by the JIRA build/release process