Add Keno's debugger instructions to the repo.

DEBUGGER.md

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+The Julia Debugger
+------------------
+
+In order to have a Julia debugger, we first need a `version of LLDB
+that understands Julia's JITed stack frames. To do so, build Julia
+with instructions from https://github.com/Keno/Cxx.jl. Just the julia
+parts are required. The Cxx part won't be necessary until the debugger
+UI is released.
+
+Use LLDB just as you would usually.
+
+This should at least get stack traces and the ability to step
+through julia code. Please file any issues as bug reports.
+
+For local variables, you may get some, and you'll get more if you set
+`DISABLE_OPT` in `src/options.h`. If you set the latter, it is useful
+to collect a set of test cases that don't get local variables even
+though they should. You will probably find a lot of functions don't
+work, yet.
+
+OS X
+====
+
+* Enable FORCE_ELF in src/options.
+
+* Comment out `if (arch.GetTriple().getVendor() != llvm::Triple::Apple)`
+in deps/llvm-svn/tools/lldb/source/Plugins/JITLoader/GDB/JITLoaderGDB.cpp
+
+* Apply http://reviews.llvm.org/D6185 if it hasn't been merged.
+
+* export `LLDB_DEBUGSERVER_PATH=/Applications/Xcode.app/Contents/SharedFrameworks/LLDB.framework/Resources/debugserver`
+
+References
+==========
+
+The original post on julia-dev: https://groups.google.com/forum/#!topic/julia-dev/gcZ5dZJni5o