adding benchmark README.txt describing how to write benchmarks

benchmark/README.txt

+The benchmarking folder contains efforts to benchmark Theano against various
+other systems.  Each subfolder corresponds to a particular type of computation,
+and each sub-subfolder corresponds to the implementation of that computation in
+with a particular software package.
+
+Since there is a variety of benchmark problems and of software systems, there
+isn't a standard for how to run the benchmark suite.
+There is however a standard for how each benchmark should produce results.
+Every benchmark run should produce one or more files with the results of
+benchmarking. These files must end with extension '.bmark'.  These files must
+have at least three lines each:
+
+1) line 1 - description of computation/problem
+2) line 2 - description of implementation/platform
+3) line 3 - time required (in seconds)
+4) line 4 - [optional] an estimated number of FLOPS performed (not necessarily same for all implementations of problem)
+