Revert "Removed generator-exercises folder as it breaks jest-codemods"

This reverts commit ecd82a4d52.

generator-exercise/generators/app/templates/README.md

@@ -0,0 +1,2 @@
+# Exercise XX - <%= title %>
+

generator-exercise/generators/app/templates/dummyfile.txt

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+<%= title %>

generator-exercise/generators/app/templates/title.js

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+let <%= title %> = function() {
+
+}
+
+module.exports = <%= title %>

generator-exercise/generators/app/templates/title.spec.js

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+let <%= title %> = require('./<%=title%>')
+
+describe('<%=title%>', function() {
+  it('EDITME', function() {
+    expect(<%=title%>()).toEqual(' ');
+  });
+
+});