Merge rabit

rabit/lib/README.md

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+Rabit Library
+=====
+This folder holds the library file generated by the compiler. To generate the library file, type ```make``` in the project root folder. If you want mpi compatible library, type ```make mpi```
+
+***List of Files***
+* rabit.a The rabit package library
+  - Normally you need to link with this one
+* rabit_mock.a The rabit package library with mock test
+  - This library allows additional mock-test
+* rabit_mpi.a The MPI backed library
+  - Link against this library makes the program use MPI Allreduce
+  - This library is not fault-tolerant
+* rabit_empty.a Dummy package implementation
+  - This is an empty library that does not provide anything
+  - Only introduced to minimize code dependency for projects that only need single machine code