make warnings

[iggysha]

Multiple warning messages when building, all related to cache:

$ make
make -C core
make[1]: Entering directory '/home/igor/dev/repo/spinel-spec/core'
docker run --rm --user=id -u:id -g -v pwd:/rfc -v /home/igor/.cache/xml2rfc:/var/cache/xml2rfc paulej/rfctools xml2rfc --text draft-rquattle-spinel-core.xml
WARNING: Unable to find a suitible cache directory to write to, trying the following directories:
/var/cache/xml2rfc
/.cache/xml2rfc
Try giving a specific directory with --cache.

[abtink]

@iggysha this whole repo is deprecated.

Please note that the spinel documentation/specification (that is used by OpenThread) is directly included in spinel.h header file from main openthread repo. The documentation/specification in this repo is not the latest and may not be fully compatible with the definitions in spinel.h (which is being used by OT between NCP/host (e.g. NCP and wpantund) and/or RCP/ot-posix).