Point out working directory space requirements

INSTALL.rst

@@ -159,17 +159,39 @@ the relevant variables and available options.
 For most installations, the default values (with no CLI options, as
 shown above) are sufficient.
 
-Next Steps
-----------
+Next Steps After Installation
+-----------------------------
 
 .. _Next Steps:
 
+Varnish working directory space requirements
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Ensure that the varnish working directory as specified by the ``-n``
+argument to ``varnishd`` has sufficient space to hold *buddy* and
+*fellow* metadata (bitmaps). It defaults to ``/var/run/varnish``, but
+is often changed by packagers to ``/var/lib/varnish``.
+
+As a rule of thumb with rounded up numbers, the amount of space needed
+in the working directory is roughly:
+
+-   5 MB for each GB of memory cache
+-  65 MB for each TB of disk cache
+
+*Note* that this is **megabytes** for gigabytes of memory and terabytes of
+disk.
+
+Continue Reading
+~~~~~~~~~~~~~~~~
+
 Now you should be ready to go.
 
 Consult the :ref:`vmod_slash(3)` and :ref:`slashmap(1)` man pages for
 detailed information on how to configure, use and monitor the SLASH/
 storage engines.
 
+The remainder of this document contains system tuning recommendations.
+
 System Setup for Optimal Performance
 ====================================