gitweb: Add info about $projectroot and $projects_list to gitweb/README

Those two configuration variables are important enough that it is
worth to explicitely write about them in the "Gitweb config file
variables" section even if they are usually set during build by
GITWEB_PROJECTROOT and GITWEB_LIST build (Makefile) configuration
variables.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

gitweb/README

@@ -129,17 +129,30 @@ descriptions.
 Gitweb config file variables
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-You can set, among others,  the following variables in gitweb config files:
+You can set, among others, the following variables in gitweb config files
+(with the exception of $projectroot and $projects_list this list does
+not include variables usually directly set during build):
  * $GIT
    Cure git executable to use.  By default set to "$GIT_BINDIR/git", which
    in turn is by default set to "$(bindir)/git".  If you use git from binary
    package, set this to "/usr/bin/git".  This can just be "git" if your
    webserver has a sensible PATH.  If you have multiple git versions
-   installed it is / can be used to choose which one to use.
+   installed it can be used to choose which one to use.
  * $version
    Gitweb version, set automatically when creating gitweb.cgi from
    gitweb.perl. You might want to modify it if you are running modified
    gitweb.
+ * $projectroot
+   Absolute filesystem path which will be prepended to project path;
+   the path to repository is $projectroot/$project.  Set to
+   $GITWEB_PROJECTROOT during installation.  This variable have to be
+   set correctly for gitweb to find repositories.
+ * $projects_list
+   Source of projects list, either directory to scan, or text file
+   with list of repositories (in the "<URI-encoded repository path> SPC
+   <URI-encoded repository owner>" format).  Set to $GITWEB_LIST
+   during installation.  If empty, $projectroot is used to scan for
+   repositories.
  * $my_url, $my_uri
    URL and absolute URL of gitweb script; you might need to set those
    variables if you are using 'pathinfo' feature: see also below.