Create README

Author: tshort

src/helpers/README.md

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+Note that the helpers defined here are used in tests, and they are useful to test out code in the REPL. 
+
+```julia
+twox(x) = 2x
+# run code in the REPL
+@jlrun twox(3)
+# compile to an executable in a `standalone` directory
+exegen([ (twox, Tuple{Int}, 4) ])
+```
+
+These are not meant to be a permanent part of the API. They are just for testing.