[similarity] adding basic double metaphone implementation

src/double_metaphone.h

@@ -0,0 +1,17 @@
+#ifndef DOUBLE_METAPHONE__H
+#define DOUBLE_METAPHONE__H
+
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct double_metaphone_codes {
+    char *primary;
+    char *secondary;
+} double_metaphone_codes_t;
+
+double_metaphone_codes_t *double_metaphone(char *input);
+
+void double_metaphone_codes_destroy(double_metaphone_codes_t *codes);
+
+#endif
+