Add a man page and an install and clean target.

Also, for good practice, don't assign but append to the cflags.

Makefile

@@ -1,7 +1,12 @@
-COMMON_CFLAGS=-Wall -Wextra -std=c99 -pedantic
-CFLAGS=`pkg-config --cflags sdl2` $(COMMON_CFLAGS)
-COMMON_LIBS=-lm
-LIBS=`pkg-config --libs sdl2` $(COMMON_LIBS)
+COMMON_CFLAGS=		-Wall -Wextra -std=c99 -pedantic
+CFLAGS+=		`pkg-config --cflags sdl2` $(COMMON_CFLAGS)
+COMMON_LIBS=		-lm
+LIBS=			`pkg-config --libs sdl2` $(COMMON_LIBS)
+PREFIX?=		/usr/local/
+INSTALL?=		install
+
+.PHONY: all
+all: Makefile sowon man
 
 sowon: main.c digits.h
 	$(CC) $(CFLAGS) -o sowon main.c $(LIBS)
@@ -11,3 +16,20 @@ digits.h: png2c digits.png
 
 png2c: png2c.c
 	$(CC) $(COMMON_CFLAGS) -o png2c png2c.c -lm
+
+docs/sowon.6.gz: docs/sowon.6
+	gzip -c docs/sowon.6 > docs/sowon.6.gz
+
+.PHONY: man
+man: docs/sowon.6.gz
+
+.PHONY: clean
+clean:
+	rm sowon docs/sowon.6.gz png2c
+
+.PHONY: install
+install: all
+	$(INSTALL) -d $(DESTDIR)/$(PREFIX)/bin
+	$(INSTALL) -C ./sowon $(DESTDIR)/$(PREFIX)/bin
+	$(INSTALL) -d $(DESTDIR)/$(PREFIX)/man/man6
+	$(INSTALL) -C docs/sowon.6.gz $(DESTDIR)/$(PREFIX)/man/man6

docs/sowon.6

@@ -0,0 +1,41 @@
+.Dd November 29, 2020
+.Dt SOWON 6
+.Os AUXILIARY
+.Sh NAME
+.Nm sowon
+.Nd Starting soon timer
+.Sh SYNOPSIS
+.Nm
+.Op Fl p
+.Op Ar seconds | Ar clock
+.Sh DESCRIPTION
+.Nm
+is a graphical countdown/timer program.
+With no arguments provided, it starts in ascending mode. With an amount of
+seconds provided, it starts in descending mode. Clock starts it in clock
+mode, that displays the current time.
+.br
+.Sh OPTIONS
+.Bl -tag -width indent
+.It Fl p
+start in paused state
+.Sh KEY BINDINGS
+.Bl -tag -width indent
+.It SPACE
+Toggle pause
+.It =
+Zoom in
+.It -
+Zoom out
+.It 0
+Zoom 100%
+.Sh FILES
+.Pa /usr/local/bin/sowon
+.br
+.Sh AUTHOR
+.An Alexey Kutepov aka. rexim
+.Aq reximkut AT gmail DOT com
+.br
+.An Source code and issue tracker at
+.Aq https://github.com/tsoding/sowon
+.P