update getting-started doc

Signed-off-by: nicksxs <nicksxs@hotmail.com>

doc/getting-started.md

@@ -12,7 +12,7 @@ If you are going to integrate ANTLR into your existing build system using mvn, a
 
 ### UNIX
 
-0. Install Java (version 1.7 or higher)
+0. Install Java (version 11 or higher)
 1. Download
 ```
 $ cd /usr/local/lib
@@ -22,6 +22,8 @@ Or just download in browser from website:
     [https://www.antlr.org/download.html](https://www.antlr.org/download.html)
 and put it somewhere rational like `/usr/local/lib`.
 
+if you are using lower version jdk, just download from [website download](https://github.com/antlr/website-antlr4/tree/gh-pages/download) for previous version, and antlr version before 4.10.1 support jdk 1.8  
+
 2. Add `antlr-4.10.1-complete.jar` to your `CLASSPATH`:
 ```
 $ export CLASSPATH=".:/usr/local/lib/antlr-4.10.1-complete.jar:$CLASSPATH"