godoc and readme update for v0.4 release

Author: jeevatkm

README.md

@@ -20,6 +20,8 @@ Simple HTTP and REST client for Go inspired by Ruby rest-client. Provides notabl
 * Choose between HTTP and RESTful mode. Default is RESTful
   * `HTTP` - default upto 10 redirects and no automatic response unmarshal
   * `RESTful` - default no redirects and automatic response unmarshal for `JSON` & `XML`
+* Custom [Root Certificates](https://godoc.org/github.com/go-resty/resty#Client.SetRootCertificate) and Client [Certificates](https://godoc.org/github.com/go-resty/resty#Client.SetCertificates)
+* Save HTTP response into File, similar to `curl -o` flag. More info [SetOutputDirectory](https://godoc.org/github.com/go-resty/resty#Client.SetOutputDirectory) & [SetOutput](https://godoc.org/github.com/go-resty/resty#Request.SetOutput).
 * Client settings like `Timeout`, `RedirectPolicy`, `Proxy` and `TLSClientConfig`
 * Client API design 
   * Have client level settings & options and also override at Request level if you want to
@@ -282,6 +284,25 @@ resp, err := resty.R().
       Post("http://myapp.com/profile")
 ```
 
+#### Save HTTP Response into File
+```go
+// Setting output directory path, If directory not exists then resty creates one!
+// This is optional one, if you're planning using absoule path in 
+// `Request.SetOutput` and can used together.
+resty.SetOutputDirectory("/Users/jeeva/Downloads")
+
+// HTTP response gets saved into file, similar to curl -o flag
+_, err := resty.R().
+          SetOutput("plugin/ReplyWithHeader-v5.1-beta.zip").
+          Get("http://bit.ly/1LouEKr")
+
+// OR using absolute path
+// Note: output directory path is not used for absoulte path
+_, err := resty.R().
+          SetOutput("/MyDownloads/plugin/ReplyWithHeader-v5.1-beta.zip").
+          Get("http://bit.ly/1LouEKr")
+```
+
 #### Request and Response Middleware
 Resty provides middleware ability to manipulate for Request and Response. It is more flexible than callback approach.
 ```go
@@ -342,6 +363,27 @@ func (c *CustomRedirectPolicy) Apply(req *http.Request, via []*http.Request) err
 resty.SetRedirectPolicy(CustomRedirectPolicy{/* initialize variables */})
 ```
 
+#### Custom Root Certificates and Client Certifcates
+```go
+// Custom Root certificates, just supply .pem file.
+// you can add one or more root certificates, its get appended
+resty.SetRootCertificate("/path/to/root/pemFile1.pem")
+resty.SetRootCertificate("/path/to/root/pemFile2.pem")
+// ... and so on!
+
+// Adding Client Certificates, you add one or more certificates
+// Sample for creating certificate object
+// Parsing public/private key pair from a pair of files. The files must contain PEM encoded data.
+cert1, err := tls.LoadX509KeyPair("certs/client.pem", "certs/client.key")
+if err != nil {
+  log.Fatalf("ERROR client certificate: %s", err)
+}
+// ...
+
+// You add one or more certificates
+resty.SetCertificates(cert1, cert2, cert3)
+```
+
 #### Proxy Settings
 Default `Go` supports Proxy via environment variable `HTTP_PROXY`. Resty provides support via `SetProxy` & `RemoveProxy`.
 Choose as per your need.

client.go

@@ -531,8 +531,9 @@ func (c *Client) SetRootCertificate(pemFilePath string) *Client {
 	return c
 }
 
-// SetOutputDirectory method sets output directory. So that HTTP response gets saved underneath
-// output directory.
+// SetOutputDirectory method sets output directory for saving HTTP response into file.
+// If the output directory not exists then resty creates one. This setting is optional one,
+// if you're planning using absoule path in `Request.SetOutput` and can used together.
 // 		resty.SetOutputDirectory("/save/http/response/here")
 //
 func (c *Client) SetOutputDirectory(dirPath string) *Client {
@@ -882,7 +883,9 @@ func (r *Request) SetAuthToken(token string) *Request {
 }
 
 // SetOutput method sets the output file for current HTTP request. Current HTTP response will be
-// saved into given file. It is similar to `curl -o` flag.
+// saved into given file. It is similar to `curl -o` flag. Absoulte path or relative path can be used.
+// If is it relative path then output file goes under the output directory, as mentioned
+// in the `Client.SetOutputDirectory`.
 // 		resty.R().
 // 			SetOutput("/Users/jeeva/Downloads/ReplyWithHeader-v5.1-beta.zip").
 // 			Get("http://bit.ly/1LouEKr")