Note addition in UserGuide, Sec. B2.2 about possible location stripping for ExternalLibraries

[Federico Cipolletta]

Following what was decided for ticket #2428, I edited the UserGuide, adding a note in order to give some details about how the ExternalLibraries' location can be stripped at compilation, in case the THORN_DIR is set to BUILD or is left empty in the .cfg file. The pull request may be found in https://bitbucket.org/cactuscode/cactus/pull-requests/98/added-one-note-about-possible/diff

[Roland Haas]

Please review.

[Roland Haas]

I added comments. At this point (306123e) it must not be applied.

[Roland Haas]

Version without PDF files and some updated text committed as git hash 878c0033 "Cactus: Add note on location stripping" of cactus