docs: Update README.md (slsa-framework#541)

Update README.md

Adding an alternative option for installing slsa-verifier if you do not rely on additional tooling. The benefit of this option is improved readability.

Signed-off-by: Drew Roen <102626803+drewroengoogle@users.noreply.github.com>
Co-authored-by: Ian Lewis <ianlewis@google.com>

README.md

@@ -154,6 +154,11 @@ $ go mod tidy
 $ cd tooling
 $ grep _ tooling_test.go | cut -f2 -d '"' | xargs -n1 -t go install
 ``` 
+Alternatively, if your project does not rely on additional tools and only uses slsa-verifier, you can instead run the following commands:
+```bash
+$ cd tooling
+$ go install github.com/slsa-framework/slsa-verifier/v2/cli/slsa-verifier
+``` 
 
 #### Option 2: Compile manually