add readme

README.md

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+# slacktime
+slacktime is a program for calculating attributes of a project network such as
+early/late start and finish times and slack.
+
+## Example
+Save this DOT file as `foo.gv`.
+```
+digraph {
+	A [DUR=10]
+	B [DUR=5]
+	C [DUR=25]
+	D [DUR=20]
+	E [DUR=50]
+	F [DUR=15]
+	G [DUR=35]
+	H [DUR=15]
+
+	A -> B
+	A -> C
+
+	B -> D
+	
+	C -> E
+	C -> F
+
+	D -> G
+	
+	E -> H
+
+	F -> H
+
+	G -> H
+}
+```
+`slacktime -f foo.gv` will output a graph with newly calculated attributes on
+each node.