Expand documentation for NewClient

Also adds a reference to the godoc for the (now old) NewClient method.

README.md

@@ -51,6 +51,8 @@ func main() {
 }
 ```
 
+If you need to provide a `context.Context` to your new client, you should use [`godo.NewClient`](https://godoc.org/github.com/digitalocean/godo#NewClient) to manually construct a client instead.
+
 ## Examples
 
 

godo.go

@@ -169,7 +169,12 @@ func NewFromToken(token string) *Client {
 	return NewClient(oauth2.NewClient(ctx, ts))
 }
 
-// NewClient returns a new DigitalOcean API client.
+// NewClient returns a new DigitalOcean API client, using the given
+// http.Client to perform all requests.
+//
+// Users who wish to pass their own http.Client should use this method. If
+// you're in need of further customization, the godo.New method allows more
+// options, such as setting a custom URL or a custom user agent string.
 func NewClient(httpClient *http.Client) *Client {
 	if httpClient == nil {
 		httpClient = http.DefaultClient