doc: add command options to README.md

Documents options accepted by node-gyp.
Continuation of nodejs#880.

README.md

@@ -144,6 +144,7 @@ Commands
 
 | **Command**   | **Description**
 |:--------------|:---------------------------------------------------------------
+| `help`        | Shows the help dialog
 | `build`       | Invokes `make`/`msbuild.exe` and builds the native addon
 | `clean`       | Removes the `build` directory if it exists
 | `configure`   | Generates project build files for the current platform
@@ -153,6 +154,34 @@ Commands
 | `remove`      | Removes the node development header files for the given version
 
 
+Command Options
+--------
+
+`node-gyp` accepts the following command options:
+
+| **Command**                       | **Description**
+|:----------------------------------|:------------------------------------------
+| `-j n`, `--jobs n`                | For `build`: run parallel builds
+| `--silly`, `--loglevel=silly`     | Log all progress to console
+| `--verbose`, `--loglevel=verbose` | Log most progress to console
+| `--silent`, `--loglevel=silent`   | Don't log anything
+| `--debug`                         | Make Debug build (default=Release)
+| `--release`, `--no-debug`         | Make Release build
+| `--make=$make`                    | Override make command, e.g. with gmake
+| `--thin=$thin`                    | 
+| `--ensure`                        | Don't reinstall headers if already present
+| `--solution=$solution`            | (windows only)
+| `--proxy=$proxy`                  |
+| `--nodedir=$path`                 | Set the path to the node binary
+| `--msys_version=$version`         | Set Visual Studio version (win)
+| `--dist-url=$url`                 |
+| `--tarball=$tarball`              |
+| `--python=$path`                  | Set path to the python (2) binary
+| `--arch=$arch`                    | Set target architecture (e.g. ia32)
+| `--cafile=$cafile`                | Override default CA chain (to download tarball)
+| `--directory=$dir`                | Run command in different directory
+
+
 License
 -------