Add multicore trace illustration to README

Author: talex5

README.md

@@ -977,6 +977,10 @@ let main ~domain_mgr =
 - : unit = ()
 ```
 
+<p align='center'>
+  <img src="./doc/traces/multicore-posix.svg"/>
+</p>
+
 Notes:
 
 - `traceln` can be used safely from multiple domains.

doc/traces/Makefile

@@ -1,4 +1,4 @@
-all: both-posix.svg cancel-posix.svg switch-mock.svg net-posix.svg
+all: both-posix.svg cancel-posix.svg switch-mock.svg net-posix.svg multicore-posix.svg
 
 %.svg: %.fxt
 	eio-trace render "$<"