doc: Rename contrib/devtools/bitcoin-tidy/README to README.md

This change fixes the file formatting on the GitHub website.

contrib/devtools/bitcoin-tidy/README.md

@@ -0,0 +1,11 @@
+# Bitcoin Tidy
+
+Example Usage:
+
+```bash
+cmake -S . -B build -DLLVM_DIR=$(llvm-config --cmakedir) -DCMAKE_BUILD_TYPE=Release
+
+cmake --build build -j$(nproc)
+
+cmake --build build --target bitcoin-tidy-tests -j$(nproc)
+```