Update README.md and license. Add examples

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

README.md

@@ -2,6 +2,8 @@
 
 go-reddit is a Go client library for accessing the Reddit API.
 
+You can view Reddit's official API documentation [here](https://www.reddit.com/dev/api/).
+
 ## Install
 
 To get a specific version from the list of [versions](https://github.com/vartanbeno/go-reddit/releases):
@@ -15,3 +17,67 @@ Or for the latest version:
 ```sh
 go get github.com/vartanbeno/go-reddit
 ```
+
+## Usage
+
+Make sure to have a Reddit app with a valid client id and secret. [Here](https://github.com/reddit-archive/reddit/wiki/OAuth2-Quick-Start-Example#first-steps) is a quick guide on how to create an app and get credentials.
+
+```go
+package main
+
+import "github.com/vartanbeno/go-reddit"
+
+func main() {
+    withCredentials := reddit.WithCredentials("id", "secret", "username", "password")
+    client, _ := reddit.NewClient(nil, withCredentials)
+}
+```
+
+The first argument (the one set to `nil`) is of type `*http.Client`. It will be used to make the requests. If nil, it will be set to `&http.Client{}`.
+
+The `WithCredentials` option sets the credentials used to make requests to the Reddit API.
+
+## Examples
+
+<details>
+    <summary>Configure the client from environment variables.</summary>
+
+```go
+client, _ := reddit.NewClient(nil, reddit.FromEnv)
+```
+</details>
+
+<details>
+    <summary>Upvote a post.</summary>
+
+```go
+_, err := client.Post.Upvote(context.Background(), "t3_postid")
+if err != nil {
+    fmt.Printf("Something bad happened: %v\n", err)
+    return err
+}
+```
+</details>
+
+<details>
+    <summary>Get a subreddit's top 5 posts of all time.</summary>
+
+```go
+result, _, err := client.Subreddit.Top(context.Background(), "golang", reddit.SetLimit(5), reddit.FromAllTime)
+if err != nil {
+    fmt.Printf("Something bad happened: %v\n", err)
+    return err
+}
+fmt.Printf("Received %d posts.\n", len(result.Posts))
+```
+</details>
+
+More examples are available in the [examples](examples) folder.
+
+## Design
+
+The package design and structure are heavily inspired from [Google's GitHub API client](https://github.com/google/go-github) and [DigitalOcean's API client](https://github.com/digitalocean/godo).
+
+## License
+
+This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details.