doc: blorb about setting npm_config_XXXX

Fixes: nodejs#1180

README.md

@@ -192,6 +192,17 @@ Command Options
 | `--solution=$solution`            | Set Visual Studio Solution version (win)
 
 
+Configuration
+--------
+
+### `node-gyp` responds to configuration settings:
+Settings take the form `npm_config_OPTION_NAME` (for any of the options listed above)
+They can be passed in two way:
+1. via `npm config` - these work only when `node-gyp` is triggered by `npm`
+   e.g. `$ npm config set npm_config_devdir /tmp/.gyp`
+2. as environment variables - work also when `noge-gyp` is invoked diretly
+   e.g. `c:\> set npm_config_devdir=c:\temp\.gyp`
+
 License
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