Add a small README explaining what is going on here

Author: PragTob

diagrams/README.md

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+# Diagrams
+
+Architecture diagrams for benchee.
+
+To render them you need a a tool such as [the PlantUML plugin for VScode](https://marketplace.visualstudio.com/items?itemName=jebbs.plantuml).
+For that to work, you also need to install `graphviz` (`sudo apt-get install graphviz`)