doc: Write about supported JDK versions

Open MPI cannot be built using JDK 7 or older anymore because of the
last commit.

Signed-off-by: KAWASHIMA Takahiro <[email protected]>

Author: kawashima-fj

README.JAVA.txt

@@ -33,6 +33,12 @@ for Open MPI Java bindings, but they were later totally rewritten.
 
 ============================================================================
 
+Supported JDK Versions
+
+Only JDK 8 and later are supported.
+
+----------------------------------------------------------------------------
+
 Building Java Bindings
 
 If this software was obtained as a developer-level
@@ -53,7 +59,7 @@ specify the location. For example, this is required on the Mac
 platform as the JDK headers are located in a non-typical location. Two
 options are available for this purpose:
 
---with-jdk-bindir=<foo> - the location of javac and javah
+--with-jdk-bindir=<foo> - the location of javac, jar, and javadoc
 --with-jdk-headers=<bar> - the directory containing jni.h
 
 For simplicity, typical configurations are provided in platform files