add a lightweight Comparator example

README.md

@@ -42,3 +42,6 @@ Of course, these same rules apply when doing `go build`, as well.
 Comparators and WriteBatch iterators must be written in C in your own
 library. This seems like a pain in the ass, but remember that you'll have the
 LevelDB C API available to your in your client package when you import levigo.
+
+An example of writing your own Comparator can be found in
+<https://github.com/jmhodges/levigo/blob/master/examples>.

examples/comparator_example.go

@@ -0,0 +1,46 @@
+package main
+
+/*
+#cgo LDFLAGS: -lleveldb
+#include <string.h>
+#include <leveldb/c.h>
+
+static void CmpDestroy(void* arg) { }
+
+static int CmpCompare(void* arg, const char* a, size_t alen,
+                      const char* b, size_t blen) {
+  int n = (alen < blen) ? alen : blen;
+  int r = memcmp(a, b, n);
+  if (r == 0) {
+    if (alen < blen) r = -1;
+    else if (alen > blen) r = +1;
+  }
+  return r;
+}
+
+static const char* CmpName(void* arg) {
+  return "foo";
+}
+
+static leveldb_comparator_t* CmpFooNew() {
+  return leveldb_comparator_create(NULL, CmpDestroy, CmpCompare, CmpName);
+}
+
+*/
+import "C"
+
+type Comparator struct {
+    Comparator *C.leveldb_comparator_t
+}
+
+func NewFooComparator() *Comparator {
+    return &Comparator{C.CmpFooNew()}
+}
+
+func (cmp *Comparator) Close() {
+    C.leveldb_comparator_destroy(cmp.Comparator)
+}
+
+func main() {
+	NewFooComparator().Close()
+}