Update README.md
Merge pull request #2 from chaubold/patch-1

Update README.md
changed how dllexport/dllimport is handled on Windows:

 * pgmlink_export.h is now static, not generated
 * cmake sets -DPGMLINK_DLL when compiling the DLL
 * don't export entire classes, but individual functions
Merge remote-tracking branch 'bekaus/master'