docs: add minimal README with screenshot

Alan Shaw · Alan Shaw · commit f98f3513edf4 · 2019-05-15T13:35:21.000+01:00
diff --git a/README.md b/README.md
@@ -0,0 +1,5 @@
+# ipfs-dag-builder-vis
+
+> See how the DAGs get built
+
+<img width="1320" alt="screenshot" src="https://user-images.githubusercontent.com/152863/57775953-28dbf380-7716-11e9-8b43-3f42b73ccc4c.png">
