documentation: Record RCU requirements

This commit adds RCU requirements as published in a 2015 LWN series.
Bringing these requirements in-tree allows them to be updated as changes
are discovered.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updates to charset and URLs as suggested by Josh Triplett. ]
7 files changed