Add Roman library

Makefile

@@ -32,7 +32,7 @@ all: \
 	id0068$(E) id0069$(E) id0070$(E) id0071$(E) id0072$(E) id0073$(E) \
 	id0074$(E) id0075$(E) id0076$(E) id0077$(E) id0078$(E) id0079$(E) \
 	id0080$(E) id0081$(E) id0082$(E) id0083$(E) id0084$(E) id0085$(E) \
-	id0086$(E) id0087$(E) id0088$(E)
+	id0086$(E) id0087$(E) id0089$(E)
 
 libeuler$(A): $(call RECURSE,lib,*.c)
 	$(RM) *.o
@@ -303,5 +303,8 @@ id0086$(E): src/id0086.c libeuler$(A)
 id0087$(E): src/id0087.c libeuler$(A)
 	$(CC) $(CFLAGS) $< -o $@ $(LEULER) $(LM)
 
+id0089$(E): src/id0089.c libeuler$(A)
+	$(CC) $(CFLAGS) $< -o $@ $(LEULER) $(LM)
+
 clean:
 	$(RM) *$(A) *.o

README.md

@@ -121,6 +121,7 @@ which is licensed under the GNU Lesser General Public License v3.0 (`LGPL-3.0`).
 | 85  | [Counting Rectangles](src/id0085.c)  |   Geometry   |     Area     |                                                                                                      |
 | 86 | [Cuboid Route](src/id0086.c) | Geometry | Minimum | |
 | 87 | [Prime Power Triples](src/id0087.c) | Number theory | Count | [Hash set](https://en.wikipedia.org/wiki/Hash_table) |
+| 89 | [Roman Numerals](src/id0089.c) | Numeral systems | Difference | [Roman numerals](https://en.wikipedia.org/wiki/Roman_numerals) |
 
 ## LeetCode