Document fscrypt cross-compiling process

Apparently cross-compiling fscrypt doesn't work:
```
$ env GOOS=linux GOARCH=arm make
go build --ldflags '-s -w -X "main.version=v0.2.3-30-g0f451a7" -X "main.buildTime=Sat Sep 1 23:10:04 CEST 2018" -extldflags ""' -o bin/fscrypt ./cmd/fscrypt
# github.com/google/fscrypt/security
security/keyring.go:157:22: undefined: getUids
security/keyring.go:159:12: undefined: setUids
security/keyring.go:163:16: undefined: setUids
# github.com/google/fscrypt/filesystem
filesystem/filesystem.go:359:15: undefined: makeLink
filesystem/filesystem.go:399:15: undefined: getMountsFromLink
make: *** [Makefile:88: bin/fscrypt] Error 2
```

This problem looks really weird since all the symbols it's complaining about are actually there.