add docs

Author: rajumsys

docs/transactional.md

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+
+# Transactional setting
+
+## Background
+As of v2.5.0 (TODO: Fix version number before release), wordpress-sparkpost introduced [Transactional](https://www.sparkpost.com/blog/transactional-email-vs-mass-email) support for emails.
+
+Emails created by WordPress are, generally, transactional in nature. Those are user registration, password reset emails etc. So, we wanted to set that flag on by default. Non-transactional emails are generated by third party plugins. In such case, that plugin should mark it as non-transactional. Because this option supports hook, it's easy to programmatically flip the value. We'll have an example below.
+
+Also because this is quite a breaking change, we won't just change this value for the existing installations. Any current installations of this plugin upgraded to new version will have this unchecked by default.
+
+## New Installations
+For new installations, Transactional will be enabled by default. In plugin's settings page, it'll be checked on. If you want to disable it, just uncheck and save the settings.
+
+## Upgrades
+If you are upgrading to this new version from an existing installation, it won't be enabled by default. You must go to plugin's setting page and enable it from there.
+
+## Programmatically modifying transactional setting
+If you know that you're sending a non-transactional email, you can set it to false before sending using a filter and then remove the filter after you sent the email. Here is an example:
+
+```php
+function unsetTransactional() {
+  return false;
+};
+
+add_filter('wpsp_transactional', 'unsetTransactional');
+
+// now send your email using wp_mail function
+
+// as sending is done, let's remove the filter so other emails are not affected
+remove_filter('wpsp_transactional', 'unsetTransactional');
+```