Install from source: Point to README.md for instructions (#4905)

Fixed GH-4697 and GH-4819

Author: AllenJB

install/unix/source.xml

@@ -70,6 +70,12 @@
   </itemizedlist>
  </para>
 
+ <simpara>
+  For more detailed steps for compiling PHP from source, see the
+  <link xlink:href="&url.php.git.src.master.view;README.md">README.md</link>
+  file in the source tarball.
+ </simpara>
+
  <simpara>
   The initial PHP setup and configuration process is controlled by the
   use of the command line options of the <command>configure</command>