Improvement ideas for README.md

One major update would be:

• explaining somewhere what all the files in the tmp folder from LIS snow cover execution are.
• as a side point, update and make more thorough the same kind of description for LIS Snow Synthesis tmp files.

Maybe two quick updates :

• in README.md, line 46, update default chain_version to 1.13 (given this issue is tagged in milestone 1.13)
• in README.md, warn users that if no water_mask is provided, no QCFLAGS file will be issued. It could be done in the Snow Product section, after the rasters definition (line 137), or in the "Be aware that" section (line 76), or in both locations. Emphasizing on the fact that not providing a TCD as input will lead to no QCFLAGS files being generated in the same section (line 78 for example) could be interesting as well.