Add note on atomic sources.

NOTES.md

@@ -4,6 +4,19 @@ A collection of stuff that we noticed during development.
 Useful later on two write a project report and to go back
 in time to find out why certain decisions where made.
 
+## 06.05.2019 - Atomics
+
+Atomics are a big part to get lock-free code running and are also
+often useful for details like counters to spin on (e.g. is a task
+finished). Resources to understand them are found in
+[a talk online](https://herbsutter.com/2013/02/11/atomic-weapons-the-c-memory-model-and-modern-hardware/)
+and in the book 'C++ concurrency in action, Anthony Williams'.
+
+The key takeaway is that we need to be careful about ordering
+possibilities of variable reads/writes. We will start of to
+use strict ordering to begin with and will have to see if this
+impacts performance and if we need to optimize it.
+
 ## 12.04.2019 - Unique IDs
 
 Assigning unique IDs to logical different tasks is key to the