cranelift: Add some filetests documentation (#4833)

cranelift/filetests/README.md

@@ -0,0 +1,36 @@
+# filetests
+
+Filetests is a crate that contains multiple test suites for testing
+various parts of cranelift. Each folder under `cranelift/filetests/filetests` is a different
+test suite that tests different parts.
+
+## Adding a runtest
+
+One of the available testsuites is the "runtest" testsuite. Its goal is to compile some piece
+of clif code, run it and ensure that what comes out is what we expect. 
+
+To build a run test you can add the following to a file:
+
+```
+test interpret
+test run
+target x86_64
+target aarch64
+target s390x
+
+function %band_f32(f32, f32) -> f32 {
+block0(v0: f32, v1: f32):
+    v2 = band v0, v1
+    return v2
+}
+; run: %band_f32(0x0.5, 0x1.0) == 0x1.5
+```
+
+Since this is a run test for `band` we can put it in: `runtests/band.clif`.
+Once we have the file in the test suite we can run it by invoking: `cargo run -- test filetests/filetests/runtests/band.clif` from the cranelift directory. 
+
+
+The first lines tell `clif-util` what kind of tests we want to run on this file. 
+`test interpret` invokes the interpreter and checks if the conditions in the `; run` comments pass. `test run` does the same, but compiles the file and runs it as a native binary. 
+
+For more information about testing see [testing.md](../docs/testing.md).