add readme for templates

tsconfig-templates/README.md

@@ -0,0 +1,5 @@
+# `tsconfig` Templates
+
+Every package should get its own copy of these four files. Package-specific options should go in `tsconfig.json` and
+test-specific options in `tsconfig.test.json`. The `cjs` and `esm` files shouldn't need to be modified, and only exist
+because tsconfigs don't support multiple inheritence.