Clarify YARA_{SRC,BUILD_*} in cross building documentation

Close hillu#117

Author: hillu

README.cross-building.md

@@ -20,6 +20,10 @@ variables need to be set when running `go build` or `go install`:
   used together with `CGO_CFLAGS` and `CGO_LDFLAGS` environment
   variables.)
 
+For the examples below, YARA is built out-of-tree. `YARA_SRC` refers
+to the directory that holds the YARA sources. The `YARA_BUILD_*`
+variables refer to directories in which build artifacts are placed.
+
 Since the MinGW environments provided by [MSYS2](https://msys2.org/)
 are technically cross-building environments, similar steps have to be
 taken, see below.