all: add .gitignore

Change-Id: I82d2a68582af37efc0618a7a52ee84aff5c71347
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2401
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..8fa3368
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+# Add no patterns to .gitignore except for files generated by the build.
+cmd/cue/cue
+**/*.test