doc: blorb about setting npm_config_OPTION_NAME

PR-URL: #1185
Fixes: #1180
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

README.md

@@ -192,6 +192,23 @@ Command Options
 | `--solution=$solution`            | Set Visual Studio Solution version (win)
 
 
+Configuration
+--------
+
+__`node-gyp` responds to environment variables or `npm` configuration__
+1. Environment variables take the form `npm_config_OPTION_NAME` for any of the 
+ options listed above (dashes in option names should be replaced by underscores).
+ These work also when `node-gyp` is invoked directly:  
+ `$ export npm_config_devdir=/tmp/.gyp`  
+ or on Windows  
+ `> set npm_config_devdir=c:\temp\.gyp`  
+2. As `npm` configuration, variables take the form `OPTION_NAME`.
+ This way only works when `node-gyp` is executed by `npm`:  
+ `$ npm config set [--global] devdir /tmp/.gyp`  
+ `$ npm i buffertools`
+
+
+
 License
 -------