gitweb: Mention optional Perl modules in INSTALL

Some optional additional Perl modules are required for some of extra
features.  Mention those in gitweb/INSTALL.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/INSTALL

@@ -237,6 +237,12 @@ Requirements
  - Perl modules: CGI, Encode, Fcntl, File::Find, File::Basename.
  - web server
 
+The following optional Perl modules are required for extra features
+ - Digest::MD5 - for gravatar support
+ - CGI::Fast and FCGI - for running gitweb as FastCGI script
+ - HTML::TagCloud - for fancy tag cloud in project list view
+ - HTTP::Date or Time::ParseDate - to support If-Modified-Since for feeds
+
 
 Example web server configuration
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~