Update README.md

This adds a short description of how WhatsApp Spoilers works and a link to the related blog post.

README.md

@@ -1,7 +1,16 @@
-# whatsapp-spoilers
+# WhatsApp Spoilers
 Send WhatsApp messages with spoiler tags!
 
 ![spoiler](spoiler.png)
 
+## About
+
+If a message is longer than a certain limit, WhatsApp clients will truncate it
+and show a "Read more" link instead. By padding a message with 4000 [zero-width
+joiners](https://en.wikipedia.org/wiki/Zero-width_joiner), we can force this
+behaviour and emulate a spoiler tag.
+
+Read the full write-up at https://blog.jiayu.co/2018/05/whatsapp-spoilers/.
+
 ## Credits
 Ji An