README: Mention the need for ngx_http_addition_module

The fancyindex_footer and fancyindex_header settings need the standard Nginx
ngx_http_addition_module built into Nginx. Add a note to this effect in the
README.

Also, a warning is issued at configuration time if the addition module is not
enabled in the configuration.

Closes issue #26.

README.rst

@@ -31,6 +31,10 @@ series onwards will work.  Note that the modules *might* compile with
 versions in the 0.6 series by applying ``nginx-0.6-support.patch``, but this
 is unsupported (YMMV).
 
+In order to use the fancyindex_header_ and fancyindex_footer_ directives
+you will also need the `ngx_http_addition_module <http://nginx.org/en/docs/http/ngx_http_addition_module.html>`_
+built into Nginx.
+
 
 Building
 ========
@@ -49,7 +53,8 @@ Building
    of the fancy indexing module::
 
     $ cd nginx-?.?.?
-    $ ./configure --add-module=../nginx-fancyindex-?.?.?  [extra desired options]
+    $ ./configure --add-module=../nginx-fancyindex-?.?.? \
+       [--with-http_addition_module] [extra desired options]
 
 4. Build and install the software::
 
@@ -141,6 +146,9 @@ fancyindex_footer
   If set to an empty string, the default footer supplied by the module will
   be sent.
 
+.. note:: Using this directive needs the ngx_http_addition_module_ built
+   into Nginx.
+
 .. warning:: When inserting custom header/footer a subrequest will be
    issued so potentially any URL can be used as source for them. Although it
    will work with external URLs, only using internal ones is supported.
@@ -159,6 +167,9 @@ fancyindex_header
   If set to an empty string, the default header supplied by the module will
   be sent.
 
+.. note:: Using this directive needs the ngx_http_addition_module_ built
+   into Nginx.
+
 fancyindex_ignore
 ~~~~~~~~~~~~~~~~~
 :Syntax: *fancyindex_ignore string1 [string2 [... stringN]]*

config

@@ -6,3 +6,7 @@ ngx_addon_name=ngx_http_fancyindex_module
 HTTP_MODULES=`echo "${HTTP_MODULES}" | sed -e \
 	's/ngx_http_index_module/ngx_http_fancyindex_module ngx_http_index_module/'`
 NGX_ADDON_SRCS="$NGX_ADDON_SRCS $ngx_addon_dir/ngx_http_fancyindex_module.c"
+
+if [ $HTTP_ADDITION != YES ] ; then
+	echo " - The 'addition' filter is needed for fancyindex_{header,footer}, but it was disabled"
+fi