Improve clarity of INSTALL

Closes #3831

INSTALL

 		      Installation Instructions
 
-Varnish uses the GNU autotools.  To build and install Varnish, simply
-run the 'configure' script in the top-level directory, then run 'make'
-and 'make install'.  On Linux, you need to run 'ldconfig' as root
-afterwards in order to update the shared library cache.
+See https://varnish-cache.org/docs/7.1/installation/install_source.html
+for complete and up to date install instructions.
 
-If you obtained the sources directly from the Git repository, you will
-need to run autogen.sh first to create the configure script.
+This file only mentions the basic steps:
+
+* Install prerequesites
+
+* When building from the source repository, run
+
+	sh autogen.sh
+
+* To build and install Varnish, run
+
+	sh configure
+	make
+	make install
 
 Varnish will store run-time state in /var/run/varnish; you may
 want to tune this using configure's --localstatedir parameter.