[master] describe usage of bin.libs

[wargio]

When bin.libs it will load all the libraries from the same folder where the selected bin is located and map them at 0x60000000000 for 64 bit archs, and at 0x60000000 for 32 bit archs.

this feature is described in librz/core/cfile.c

also talking about dir.libs=/path/to/my/folder is also a good idea