Add a README with instructions for developing the package locally (#55)

README.md

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+# Statistics.jl
+
+Development repository for the Statistics standard library (stdlib) that ships with Julia. 
+
+#### Using the development version of Statistics.jl
+
+If you want to develop this package, do the following steps:
+- Clone the repo anywhere.
+- In line 2 of the `Project.toml` file (the line that begins with `uuid = ...`), modify the UUID, e.g. change the `107` to `207`.
+- Change the current directory to the Statistics repo you just cloned and start julia with `julia --project`.
+- `import Statistics` will now load the files in the cloned repo instead of the Statistics stdlib.
+- To test your changes, simply do `include("test/runtests.jl")`.
+
+If you need to build Julia from source with a git checkout of Statistics, then instead use `make DEPS_GIT=Statistics` when building Julia. The `Statistics` repo is in `stdlib/Statistics`, and created initially with a detached `HEAD`. If you're doing this from a pre-existing Julia repository, you may need to `make clean` beforehand.