readme: update state diagram

Redraw state diagram with https://github.com/mermaid-js/mermaid .

Closes #148

Author: LeonidVas

README.md

@@ -321,7 +321,22 @@ For information on the transition triggers, refer to:
 * description of the `timeout`, `ttl timeout`, and `ttr timeout` options in
 the sections of the corresponding [queue types](#queue-types).
 
-![Task state diagram](./doc/images/statediagram.svg)
+```mermaid
+flowchart LR
+      INIT((" "))-->  |"put()"| READY
+      INIT((" "))--> |"put('my_task_data', {delay = delay})"| DELAYED
+      READY--> |"take()"| TAKEN
+      READY--> |"delete() / ttl timeout"| DONE
+      READY--> |"bury()"| BURIED
+      TAKEN--> |"release() / ttr timeout"| READY
+      TAKEN--> |"release\n(id, {delay = delay})"| DELAYED
+      TAKEN--> |"ack() / delete()"| DONE
+      TAKEN--> |"bury()"| BURIED
+      BURIED--> |"delete() /\nttl timeout"| DONE
+      BURIED--> |"kick()"| READY
+      DELAYED--> |"timeout"| READY
+      DELAYED--> |"delete()"| DONE
+```
 
 # Installing