Add notes on sources of prefix-sum algorithm

NOTES.md

@@ -4,6 +4,12 @@ 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.
 
+## 11.06.2019 - Parallel Scan
+
+Our parallel scan is oriented on the
+['shared memory: two level algorithm'](https://en.wikipedia.org/wiki/Prefix_sum)
+and our understanding is using [slides from a cs course](http://www.cs.princeton.edu/courses/archive/fall13/cos326/lec/23-parallel-scan.pdf).
+
 ## 06.05.2019 - Relaxed Atomics
 
 At the end of the atomic talk it is mentioned how the relaxed