Establish OpenJDK 9+181 as an upstream revision of interest.

jdk-9+181 (added 3 August 2017) is currently the latest
tag at http://hg.openjdk.java.net/jdk9/jdk9/

This CL arbitrarily picks that commit as an initial reference
point, from which one may want to integrate upstream sources.
This tag should be pretty close to the public availability build
of OpenJDK 9, but I don't know whether it is exactly identical.

This CL adds toplevel documentation to CompareUpstream for how to
check out that upstream revision, but does not change the expected
upstream version of any particular ojluni class and doesn't change
the tool to compare ojluni files against the new upstream.

The purpose of this CL is merely to pick, and name, a new upstream
revision that other CLs may want to refer to.

Test: Ran the given commands to check out upstream sources.
Test: Treehugger.
Change-Id: I89e95a98268d38224f663c449f019624f8d1c685