makefile: add PREFIX and ensure_bin target

- add build/install instructions

Makefile

@@ -7,7 +7,12 @@ CFLAGS=-W -O
 LDFLAGS=
 LDLIBS=
 
-all: journal
+PREFIX=/usr/local
+
+all: ensure_bin journal
+
+ensure_bin: bin
+	mkdir -p bin
 
 install: journal
 	mkdir -p $(DESTDIR)$(PREFIX)/bin
@@ -16,9 +21,8 @@ install: journal
 uninstall:
 	rm $(DESTDIR)$(PREFIX)/bin/journal
 
-# builds for the journal only, other utils might come on later
+# currently we only have one journal util, others may come later
 journal: src/journal/main.o
-	mkdir -p bin
 	$(CC) $(LDFLAGS) -o bin/journal src/journal/main.o $(LDLIBS)
 
 src/journal/main.o: src/journal/main.c
@@ -28,4 +32,6 @@ clean:
 
 .SUFFIXES: .c .o
 .c.o:
+	# $< mean the prereqs
+	# $@ means the target
 	$(CC) $(CFLAGS) -c $< -o $@

README.md

@@ -1,3 +1,16 @@
 # lunabot
 
-journal-like util for me
+journal-like util for me
+
+## building
+
+ - a robot girl
+ - posix compliant make
+ - gcc/any cc
+
+```
+make
+
+# or just remove PREFIX for /usr/local
+make PREFIX=$HOME/.local install
+```