Add the Java example

Signed-off-by: Vadim Markovtsev <vadim@sourced.tech>

PublicGitArchive/examples/java.md

@@ -0,0 +1,8 @@
+Downloading all Java repositories
+=================================
+
+Execute the following to download all Java siva files to the current directory:
+
+```bash
+./multitool get-index | grep -P '[",]Java[",]' | grep -oP '[0-9a-f]{40}\.siva' | ./multitool get-dataset -o .
+```