Add description for CHECK_DIAG flag

INSTALL.md

@@ -257,7 +257,7 @@ Features useful to deal with legacy systems
  * `-D__HAS_NO_OMP_3` CP2K assumes that compilers support OpenMP 3.0. If this is not the case specify this flag to compile. Runtime performance will be poorer on low numbers of processors
  * `-D__HAS_NO_CUDA_STREAM_PRIORITIES` - Needed for CUDA sdk version < 5.5
  * `-D__NO_STATM_ACCESS` - Do not try to read from /proc/self/statm to get memory usage information. This is otherwise attempted on several. Linux-based architectures or using with the NAG, gfortran, compilers.
  * `-D__F2008` Allow for conformity check with the Fortran 2008 standard when using the GFortran compiler flag `-std=f2008`
  * `-D__CHECK_DIAG` Debug option which activates an orthonormality check of the eigenvectors calculated by the selected eigensolver

## 4. If it doesn't work?
If things fail, take a break... go back to 2a (or skip to step 6).

src/cp2k_info.F

@@ -286,6 +286,9 @@ FUNCTION cp2k_flags() RESULT(flags)
#if defined __SIRIUS
      flags = TRIM(flags)//" sirius"
#endif
#if defined __CHECK_DIAG
      flags = TRIM(flags)//" check_diag"
#endif

   END FUNCTION cp2k_flags