Add description of available env vars.

Author: makasim

README.md

@@ -9,4 +9,22 @@ There must be a `app.php` inside the folder.
 
 ```
 docker run --rm -it --env NGINX_WEB_ROOT=/app/web -p 80:80 -v /home/user/app/:/app makasim/nginx-php-fpm
-```
+```
+
+## Env vars
+
+Here's available env vars and their default values:
+
+```
+NGINX_WEB_ROOT=     /var/www/html
+NGINX_PHP_FALLBACK= /app.php
+NGINX_PHP_LOCATION= ^/app\.php(/|$)
+NGINX_USER=         www-data
+NGINX_CONF=         /etc/nginx/nginx.conf
+
+PHP_SOCK_FILE=      /run/php.sock
+PHP_USER=           www-data
+PHP_GROUP=          www-data
+PHP_MODE=           0660
+PHP_FPM_CONF=       /etc/php/7.0/fpm/php-fpm.conf
+```