Update README.md

Author: easterncoder

README.md

@@ -64,23 +64,39 @@ docker-compose stop
 
 Build options are specified by passing --build-arg FOO=BAR for each argument you want to pass. Available build arguments are:
 
-Host UID (required)
-- `HOST_UID`=num\
-Example 1: to set the current user's UID `--build-arg HOST_UID=$(id -u)`
+`HOST_UID`=num\ (required)
+
+The HOST_UID is used to ensure that the host user and the docker www-data user have the same User ID and keep permissions the same. You'd usually want to use the same user ID as the current logged in user in the host in which case you can just do set the value to `$(id -u)` which should work in most \*NIX systems.
+
+Example 1: to set the current user's UID `--build-arg HOST_UID=$(id -u)`\
 Example 2: to set a specific UID `--build-arg HOST_UID=1001`
 
-Web Servers:
-- `WEB`=**httpd** | nginx
-- `APP_HOSTNAME`=**myapp** | _string_
+- `APP_HOSTNAME`=**myapp** | _string_ (optional)
+
+The host name that you want to use for for your application. This makes your web server accessible via **appname.local** on your host machine. Default value is **myapp** which makes the web server accessible via **myapp.local**
+
+- `WEB`=**httpd** | nginx (optional)
+
+The web server that you want to use. Available values are httpd (apache) or nginx. Default is httpd.
+
+- `DB`=**mariadb** | mysql (optional)
 
-Database Management System:
-- `DB`=**mariadb** | mysql
-- `DB_VERSION`=**latest** | _num_
+The DBMS that you want to use. Default is **mariadb** but you can also choose mysql.
+
+Warning: Switching DBMS on an existing container is most likely to cause errors due to incompatibility in data formats. It's best to dump the database first, clear the contents of `app/db`, switch DBMS and reimport the sql dump.
+
+- `DB_VERSION`=**latest** | _num_ (optional)
+
+The DBMS version that you want to use. Default version is **latest**. For a list of available versions, see https://hub.docker.com/_/mariadb and https://hub.docker.com/_/mysql.
+
+Warning: Switching DBMS on an existing container is most likely to cause errors due to incompatibility in data formats. It's best to dump the database first, clear the contents of `app/db`, switch DBMS and reimport the sql dump.
 
-PHP
 - `PHP_VERSION`=**7** | 7.4 | 7.3 | 7.2 | 7.1 | 7.0 | 5.6
 
-To build PHP 5.6, Nginx (latest) and MySQL 5.6, the build command would be:
+The PHP version that you want to use. Currently supports 5.6, 7.0, 7.1, 7.2, 7.3 and 7.4. Setting the version to just *7* will setup the latest PHP 7 available (ie 7.4)
+
+
+Build Example: To build PHP 5.6, Nginx (latest) and MySQL 5.6, the build command would be:
 
 ```sh
 docker-compose build --build-arg HOST_UID=$(id -u) \