doc: blorb about setting `npm_config_OPTION_NAME`
doc: blorb about setting `npm_config_OPTION_NAME`

PR-URL: https://github.com/nodejs/node-gyp/pull/1185
Fixes: https://github.com/nodejs/node-gyp/issues/1180
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Gibson Fahnestock <c0a850ada395a21aea35ea96f1fadcdd1d6eb48b@gmail.com>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: Anna Henningsen <2bc1ecb410e142bce83bce6f212b41e1781536dc@addaleax.net>
nwjs/nw-gyp,TooTallNate/node-gyp,nodegit/node-gyp,nodejs/node-gyp,isaacs/node-gyp,nodejs/node-gyp,nodejs/node-gyp,TooTallNate/node-gyp,isaacs/node-gyp,TooTallNate/node-gyp,nwjs/nw-gyp,refack/node-gyp,nodegit/node-gyp,refack/node-gyp,saper/node-gyp,isaacs/node-gyp,saper/node-gyp,isaacs/node-gyp,refack/node-gyp,nwjs/nw-gyp,isaacs/node-gyp,nodejs/node-gyp,saper/node-gyp,nodegit/node-gyp,TooTallNate/node-gyp,nodegit/node-gyp,TooTallNate/node-gyp,saper/node-gyp,nodejs/node-gyp,nwjs/nw-gyp,refack/node-gyp,saper/node-gyp,refack/node-gyp,nwjs/nw-gyp,nodegit/node-gyp