UPDATING: Add a note about running installworld twice

Some folks seem to be hitting issues with build orchestration; presumably
some of our .WAIT-removal optimizations are going awry, and they're ending
up with applications linked against new libc being installed before the new
libc.

Letting installworld complete the first time should ensure that the new libc
is installed by the end of it, then the second installworld will ensure
consistency as everything should succeed.