[Misc] Update README.md by removing installation approaches unavailable for now

Summary: Update README.md by removing installation approaches unavailable for now

Test Plan: N/A

Reviewed-by: sanhong

Issue: #9

README.md

@@ -14,38 +14,20 @@ Alibaba Dragonwell JDK currently supports Linux/x86_64 platform only.
 
 ### Installation
 
-##### Option 1, Download and install pre-built Alibaba Dragonwell
-
 * You may download a pre-built Alibaba Dragonwell JDK from its GitHub page:
 https://github.com/alibaba/dragonwell11/releases.
 * Uncompress the package to the installation directory.
 
-##### Option 2, Install via YUM
-
-Alibaba Dragonwell is officially supported and maintained in Alibaba Cloud Linux 2 (Aliyun Linux 2) YUM repository, and this repo should be also compatible with Aliyun Linux 17.1, Red Hat Enterprise Linux 7 and CentOS 7.
-
-* For users running Alibaba Cloud Linux 2 OS, you should be able to install Alibaba Dragonwell by simply running: `sudo yum install -y java-11-alibaba-dragonwell`;
-* For users running with aforementioned compatible distros, place a new repository file under `/etc/yum.repos.d` (e.g.: `/etc/repos.d/alinux-plus.repo`) with contents as follows, then you should be able to install Alibaba Dragonwell by executing: `sudo yum install -y java-11-alibaba-dragonwell`:
-```
-# plus packages provided by Aliyun Linux dev team
-[plus]
-name=AliYun-2.1903 - Plus - mirrors.aliyun.com
-baseurl=http://mirrors.aliyun.com/alinux/2.1903/plus/$basearch/
-gpgcheck=1
-gpgkey=http://mirrors.aliyun.com/alinux/RPM-GPG-KEY-ALIYUN
-```
-
 ### Enable Alibaba Dragonwell for Java applications
 
 To enable Alibaba Dragonwell JDK for your application, simply set `JAVA_HOME` to point to the installation directory of Alibaba Dragonwell. If you installed Dragonwell JDK via YUM, follow the instructions prompted from post-install outputs, e.g.:
 
 ```
 =======================================================================
-Alibaba Dragonwell is installed to:
-    /opt/alibaba/java-11-alibaba-dragonwell-11.0.5.1-Dragonwell+10.al7
-You can set Alibaba Dragonwell as default JDK by exporting the
-following ENV VARs:
-$ export JAVA_HOME=/opt/alibaba/java-11-alibaba-dragonwell-11.0.5.1-Dragonwell+10-preview-a1.al7
+# Assuming Alibaba Dragonwell 11 is installed to:
+#    /opt/alibaba/java-11-alibaba-dragonwell
+# You can set Alibaba Dragonwell as default JDK by exporting following environment variables:
+$ export JAVA_HOME=/opt/alibaba/java-11-alibaba-dragonwell
 $ export PATH=${JAVA_HOME}/bin:$PATH
 =======================================================================
 ```