more default docs.

common/src/tectonic/resources/Map.md

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+# Map
+
+A map is a data structure that "maps" one type (keys) to another (values).
+
+Examples:
+* Mapping [Strings](./String) to [Strings](./String)
+  ```yml
+  a: "thing"
+  b: "thing 2"
+  c: "thing 3"
+  ```
+* Mapping [Strings](./String) to [Integers](./Integer)
+  ```yml
+  a: 1
+  b: 2
+  c: 3
+  ```