Without this, the build fails with the following error:

	/usr/bin/ld: /tmp/cch2zAHq.o: undefined reference to symbol 'sincos@@GLIBC_2.2.5'
	/usr/bin/ld: /usr/lib/libm.so.6: error adding symbols: DSO missing from command line
	collect2: error: ld returned 1 exit status
---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 86469bc..6fe4fa6 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@ A [vector editor](https://wiki.xxiivv.com/dotgrid), written in ANSI C.
 You must have [SDL2](https://wiki.libsdl.org/).
 
 ```
-cc dotgrid.c -std=c89 -Os -DNDEBUG -g0 -s -Wall -L/usr/local/lib -lSDL2 -o dotgrid
+cc dotgrid.c -std=c89 -Os -DNDEBUG -g0 -s -Wall -L/usr/local/lib -lSDL2 -lm -o dotgrid
 ```
 
 ## I/O
-- 
2.34.1