added more resources to README, so this repo can serve as my full-on learning path and reinforcement learning... package? added some more chapters, removed ch3 entirely.

ch6.md

@@ -0,0 +1,4 @@
+// make an Enum and use it
+// make an Enum one of whose variants' types is itself an Enum
+// make an Enum one of whose variants' takes an argument, then use that argument in a match (expression?)
+// use a catchall in a match expression