search: add a tip about negated searches (#720)

Author: LawnGnome

cmd/src/search.go

@@ -49,6 +49,12 @@ Other tips:
   Force color output on (not on by default when piped to other programs) by setting COLOR=t
 
   Query syntax: https://about.sourcegraph.com/docs/search/query-syntax/
+
+  Be careful with search strings including negation: a search with an initial
+  negated term may be parsed as a flag rather than as a search string. You can
+  use -- to ensure that src parses this correctly, eg:
+
+    	$ src search -- '-repo:github.com/foo/bar error'
 `
 
 	flagSet := flag.NewFlagSet("search", flag.ExitOnError)