Add README to multi-setup-runtime

Author: chillenzer

examples/multi-setup-runtime/README.md

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+# Multi setup runtime
+
+This examples shows how to use multiple, runtime-parametrised setups.
+In this case, we're building on top of the `plain-malloc` example,
+generalising to multiple different allocation sizes.
+
+The interface to do so is not yet the most beautiful one
+and likely (or hopefully) there will be change in the near future.