Mention install requirements

INSTALL.rst

@@ -39,6 +39,28 @@ installed, and the auto-tools will attempt to locate the Varnish
 instance, and then pull in libraries and other support files from
 there.
 
+Requirements
+------------
+
+.. _`Varnish-Cache build dependencies`: https://varnish-cache.org/docs/trunk/installation/install_source.html
+
+We recommend to just install the `Varnish-Cache build dependencies`_
+for the system on which this vmod is to be built.
+
+The following packages are required at a minimum::
+
+  make
+  automake
+  autoconf
+  libpcre2-dev
+  libtool
+  pkg-config
+
+In particular, ``libpcre2-dev`` refers to a PCRE2 installation with
+headers.
+
+Your system might use slightly different names.
+
 Quick start
 -----------