Added Build and usage instructions

Build and usage instructions
How to, step by step:

1. Generate lex.yy.c by running flex in command line:
hit_compilers_project\source files>flex exprs.lex

2. Build the solution located in hit_compilers_project\vs2010\mvcompiler
using Visual Studio 2010

3. Compile your language in command line, for example:
hit_compilers_project\vs2010\mvcompiler\Debug\mvcompiler.exe <
"_FULLPATH_\hit_compilers_project\Results of
execution\input_correct_2.tx
t"