8251551: Use .md filename extension for README

README.md

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+# README:
+
+This file should be located at the top of the OpenJDK Mercurial root
+repository. A full OpenJDK repository set (forest) should also include
+the following 6 nested repositories:
+"jdk", "hotspot", "langtools", "corba", "jaxws"  and "jaxp".
+
+The root repository can be obtained with something like:
+
+```
+hg clone http://hg.openjdk.java.net/jdk8/jdk8 openjdk8
+```
+
+You can run the get_source.sh script located in the root repository to get
+the other needed repositories:
+
+```
+cd openjdk8 && sh ./get_source.sh
+```
+
+People unfamiliar with Mercurial should read the first few chapters of
+the Mercurial book: <http://hgbook.red-bean.com/read/>
+
+See <http://openjdk.java.net/> for more information about OpenJDK.
+
+## Simple Build Instructions:
+
+1. Get the necessary system software/packages installed on your system, see
+    <http://hg.openjdk.java.net/jdk8/jdk8/raw-file/tip/README-builds.html>
+
+1. If you don't have a jdk7u7 or newer jdk, download and install it from
+    <http://java.sun.com/javase/downloads/index.jsp>
+    Add the /bin directory of this installation to your PATH environment
+    variable.
+
+1. Configure the build:
+    ```
+    bash ./configure
+    ```
+
+1. Build the OpenJDK:
+    ```
+    make all
+    ```
+    The resulting JDK image should be found in `build/*/images/j2sdk-image`
+
+where make is GNU make 3.81 or newer, /usr/bin/make on Linux usually
+is 3.81 or newer. Note that on Solaris, GNU make is called "gmake".
+
+Complete details are available in the file:
+<http://hg.openjdk.java.net/jdk8/jdk8/raw-file/tip/README-builds.html>