Create buddypress-fix.md

Add doc for buddypress compatibility fix

Author: rajumsys

docs/buddypress-fix.md

@@ -0,0 +1,11 @@
+# Send Buddypress emails using SparkPost 
+
+Buddypress plugin implements their own mailer (`bb_mail`) by creating a seperate mailer object of phpMailer class; same way SparkPost plugin does for HTTP Mailer. That's why they don't work nicely with each other out-of-the-box. However, Buddypress developers also made it extremely easy to fallback to default `wp_mail`. 
+
+To fix the problem, just add the following snippet in your theme's `functions.php` (feel free to create, if one doesn't exist already). 
+
+```
+add_filter('bp_email_use_wp_mail', function() {
+  return true;
+});
+```