symbiflow: add README file

Signed-off-by: Alessandro Comodi <[email protected]>

Author: acomodi

vtr_flow/arch/symbiflow/README.md

@@ -0,0 +1,12 @@
+SymbiFlow architectures
+=======================
+
+This directory holds all the SymbiFlow architectures that are generated in the [SymbiFlow-arch-defs](https://github.com/SymbiFlow/symbiflow-arch-defs) repository.
+
+The data files needed to successfully run VPR are:
+- Architecture XML definition
+- RR Graph
+- Router Lookahead
+- Place delay matrix lookup
+
+All the data files can be downloaded with the `make get_symbiflow_benchmarks` target in the root directory.