Add exercise README templates

We are working towards making exercises stand-alone—that is to say:
no more generating READMEs on the fly.

This will give maintainers more control over each individual exercise
README.

The template uses the Go text/template package, and the default templates
generate the exact same READMEs as we have been generating on the fly.

config/exercise_readme.go.tmpl

@@ -0,0 +1,16 @@
+# {{ .Spec.Name }}
+
+{{ .Spec.Description -}}
+{{- with .Hints }}
+{{ . }}
+{{ end }}
+{{- with .TrackInsert }}
+{{ . }}
+{{ end }}
+{{- with .Spec.Credits -}}
+## Source
+
+{{ . }}
+{{ end }}
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.