Add php_fastcgi docs

v2:-Documentation.md

@@ -37,6 +37,7 @@ Features which are available in Caddy Enterprise are indicated with    🏢
 				- [file_server](#file_server)
 				- [encode](#encode)
 				- [headers](#headers)
+				- [php_fastcgi](#php_fastcgi)
 				- [reverse_proxy](#reverse_proxy)
 				- [redir](#redir)
 				- [rewrite](#rewrite)
@@ -520,6 +521,37 @@ headers [<matcher>] [[+|-]<field> <value>] {
 }
 ```
 
+##### php_fastcgi
+
+Proxies requests to one or more FastCGI backends specially configured for PHP apps. It has the same syntax and options as [reverse_proxy](#reverse_proxy), but is shorthand for a configuration equivalent to this example:
+
+```
+# internally rewrite URIs to match index.php files
+matcher indexFiles {
+    file {
+        try_files {path} index.php
+    }
+}
+rewrite match:indexFiles {http.matchers.file.relative}
+
+# proxy any requests for PHP files to backend via FastCGI
+matcher phpFiles {
+    path *.php
+}
+reverse_proxy match:phpFiles php-fpm:9000 {
+    transport fastcgi {
+        split .php
+    }
+}
+```
+
+Thus, the following single line is sufficient and works for _most_ PHP scripts (replace php-fpm:9000 with your php-fpm listener):
+
+```
+php_fastcgi php-fpm:9000
+```
+
+
 ##### reverse_proxy
 
 Proxies requests to one or more backends with configurable transport options.
@@ -580,6 +612,8 @@ transport fastcgi {
 }
 ```
 
+For proxying to PHP via FastCGI (php-fpm) specifically, see the [php_fastcgi](#php_fastcgi) directive.
+
 ##### redir
 
 Issue an HTTP redirect.