Documentation: Include Java 8 requirement in installation notes

Bug: Issue 7843
Change-Id: I763800eab693c357eace4182c3bafa28c622b94d

Documentation/install.txt

diff --git a/Documentation/install.txt b/Documentation/install.txt
index 2adc343..87d757e 100644
--- a/Documentation/install.txt
+++ b/Documentation/install.txt
@@ -5,7 +5,9 @@
 To run the Gerrit service, the following requirements must be met on
 the host:
 
-* JRE, minimum version 1.8 http://www.oracle.com/technetwork/java/javase/downloads/index.html[Download]
+* JRE, version 1.8 http://www.oracle.com/technetwork/java/javase/downloads/index.html[Download]
++
+Gerrit is not yet compatible with Java 9 or newer at this time.
 
 You'll also need an SQL database to house the review metadata. You have the
 choice of either using the embedded H2 or to host your own MySQL or PostgreSQL.