Add essential environment variables to README files

[Ian Culverwell]

ROPP10.0 beta reviewers recommend

The only explanation which environment variables must be set for 
successful installation are in the Release Notes; they should be more 
easily available, e.g. in the README files of each sub-package.

Sounds fairy nuff.

[warrick]

Following the ROPP 10.0 beta review, the environment variable ECCODES_BUFR_SET_TO_MISSING_IF_OUT_OF_RANGE is now set to 1 from within ropp2bufr_eccodes and eum2bufr_eccodes, so it doesn't need to be set by the user.

This fact is now mentioned in README_v100.deps, the ROPP IO UG, as well is in the Version 10.0 Release Notes

[warrick]

The ecCodes documentation updates were made at r6401 (ROPP IO UG), r6388 (README_v100.deps) and r6389 (v10 Release Notes)

[warrick]

It is also recommended to set some environment variables for the ROPP_IO and ROPP_PP modules, but this was not fully explained the documentation, so it is now mentioned in:

The READMEs within ROPP_PP and ROPP_IO (r6438)

The v10 Release Notes (IO variables were mentioned, but not PP) r6437

[warrick]

Correction - my reference to r6401 above should be r6411

[warrick]

Updated ROPP_IO and ROPP_PP READMEs at r6447 and r6448 respectively

[Ian Culverwell]

README files further polished at r6460.

[Ian Culverwell]

Release Notes further polished at r6462.