chore: add reuse copyright/license badge to README (#692)

This will use FSFE API server to validate main branch is reuse compliant
fixes #684

Author: kbroch-rivosinc

README.adoc

@@ -1,5 +1,8 @@
 = RISC-V Unified Database (UnifiedDB/UDB)
 
+image::https://api.reuse.software/badge/github.com/riscv-software-src/riscv-unified-db[REUSE status, link="https://api.reuse.software/info/github.com/riscv-software-src/riscv-unified-db"]
+
+
 [WARNING]
 ----
 This project is under rapid development. Expect schemas and APIs to change.