Describe undesirable but necessary cruft

cruft.md

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+As of Ruby 1.9.3, it is impossible to dynamically generate a Symbol
+through interpolation without generating garbage. Theoretically, Ruby
+should be able to take care of this by building up the String in C and
+interning the C String.
+
+Because of this, we avoid generating dynamic Symbols at runtime. For
+example, instead of generating the instrumentation event dynamically, we
+have a constant with a Hash of events:
+
+```ruby
+INSTRUMENT = {
+  :serialize => :"serialize.serializer",
+  :associations => :"associations.serializer"
+}
+```
+
+If Ruby ever fixes this issue and avoids generating garbage with dynamic
+symbols, this code can be removed.
+