[TASK] Added Documentation

README.md

+# sg_mail
+
+This extension provides an email templating service and mail queue for other extensions.
+Templates in various languages can be managed in the backend. Additionally sg_mail provides a finisher class
+for the Formhandler extension, making it possible to manage formhandler templates in the backend.
+
+## Registering your Extension/Plugin 
+
+To integrate sg_mail available for your extension, you have to register it inside your **ext_localconf.php**
+by supplying a path string to an implementation of **SGalinski\SgMail\Service\RegisterInterface** with:
+
+* your extension key
+* a name for the template
+* the path to your default template files (template files should be named *language_code*.template.html. Example: en.template.html)
+* a brief description of the template (when is it used ?)
+* the template markers(placeholder variables)
+* the subjects for all templates. Here you have to use an associative array language_code => subject
+
+### Example :
+
+An example class can be found here: **SGalinski\SgMail\Example\Register**
+
+Register your class in **ext_localconf.php** with:
+
+```\SGalinski\SgMail\Service\MailTemplateService::registerByFile('SGalinski\SgMail\Example\Register');```
+
+### Send an email with the MailTemplateService
+
+Basic Usage:
+
+1. Get an instance of MailTemplateService
+2. Set all desired fields (i.e. setLanguage, setToAddresses, setTemplateName, setExtensionKey etc.)
+3. Invoke the **sendEmail()** function
+
+You can also set the priority (setPriority function of the service) for your mails with the constants 
+of the **SGalinski\SgMail\Domain\Model\Mail** class. With the Flag **ignoreMailQueue** you can switch between
+queueing your Mail(false) or immediately(true) sending it.
+
+* PRIORITY_LOWEST
+* PRIORITY_LOW
+* PRIORITY_MEDIUM
+* PRIORITY_HIGH
+* PRIORITY_HIGHEST
+
+The higher the priority, the more likely the mail will get send immediately (depending how "busy" the MailQueue is).
+
+## Using sg_mail with Formhandler
+
+In your **setup.ts** of your formhandler extension, you have to invoke the **FormhandlerFinisherService** of sg_mail.
+This is done in a similar fashion as with the usual finisher classes.
+
+### Example of sg_formhandler_ordercard
+
+```
+finishers {
+		# Finisher_Mail sends emails to an admin and/or the user.
+		1 {
+			class = SGalinski\SgMail\Service\FormhandlerFinisherService
+			config {
+				checkBinaryCrLf = message
+				template_key = order_card_admin
+				extension_key = sg_formhandler_ordercard
+				to_address = admin@sgalinski.de
+				from_address = info@sgalinski.de
+				from_name = sgalinski.de
+				ignore_mail_queue = TRUE
+			}
+		}
+}
+```