Document cross platform options (#130)

Author: fishbone1

README.md

@@ -37,6 +37,8 @@ prebuild-install [options]
   --runtime     -r  runtime     (Node runtime [node, napi or electron] to build or install for, default is node)
   --path        -p  path        (make a prebuild-install here)
   --token       -T  gh-token    (github token for private repos)
+  --arch            arch        (target CPU architecture, see Node OS module docs, default is current arch)
+  --platform        platform    (target platform, see Node OS module docs, default is current platform)
   --tag-prefix <prefix>         (github tag prefix, default is "v")
   --build-from-source           (skip prebuild download)
   --verbose                     (log verbosely)
@@ -46,8 +48,8 @@ prebuild-install [options]
 ```
 
 When `prebuild-install` is run via an `npm` script, options
-`--build-from-source`, `--debug` and `--download`, may be passed through via
-arguments given to the `npm` command.
+`--build-from-source`, `--debug`, `--download`, `--target`, `--runtime`, `--arch` and `--platform` may be passed through via
+arguments given to the `npm` command. Alternatively you can set environment variables `npm_config_platform`, `npm_config_arch`, `npm_config_target` and `npm_config_runtime`.
 
 ### Private Repositories
 

help.txt

@@ -5,6 +5,8 @@ prebuild-install [options]
   --runtime     -r  runtime     (Node runtime [node or electron] to build or install for, default is node)
   --path        -p  path        (make a prebuild-install here)
   --token       -T  gh-token    (github token for private repos)
+  --arch            arch        (target CPU architecture, see Node OS module docs, default is current arch)
+  --platform        platform    (target platform, see Node OS module docs, default is current platform)
   --tag-prefix <prefix>         (github tag prefix, default is "v")
   --force                       (always use prebuilt binaries when available)
   --build-from-source           (skip prebuild download)