README: Suggest working with Go packages by default.

When writing Go code that is something more than just a very short snippet, it helps to organize code as a Go package. It may contain 1 or more .go files in a directory.

`go build` is primarily used with Go packages, as in `go build import/path`. We should suggest the same use of `gopherjs build`. That will make it easier for users to get started and grow their code, because they'll be able to split into multiple .go files without having to update their build step.

README.md

@@ -15,7 +15,7 @@ Get or update GopherJS and dependencies with:
 go get -u github.com/gopherjs/gopherjs
 ```
 
-Now you can use  `gopherjs build [files]` or `gopherjs install [package]` which behave similar to the `go` tool. For `main` packages, these commands create a `.js` file and `.js.map` source map in the current directory or in `$GOPATH/bin`. The generated JavaScript file can be used as usual in a website. Use `gopherjs help [command]` to get a list of possible command line flags, e.g. for minification and automatically watching for changes. If you want to run the generated code with Node.js, see [this page](https://github.com/gopherjs/gopherjs/blob/master/doc/syscalls.md).
+Now you can use  `gopherjs build [package]` or `gopherjs install [package]` which behave similar to the `go` tool. For `main` packages, these commands create a `.js` file and `.js.map` source map in the current directory or in `$GOPATH/bin`. The generated JavaScript file can be used as usual in a website. Use `gopherjs help [command]` to get a list of possible command line flags, e.g. for minification and automatically watching for changes. If you want to run the generated code with Node.js, see [this page](https://github.com/gopherjs/gopherjs/blob/master/doc/syscalls.md).
 
 *Note: GopherJS will try to write compiled object files of the core packages to your $GOROOT/pkg directory. If that fails, it will fall back to $GOPATH/pkg.*