Add more content to the Readme. (#73)

* Add more content to the Readme.

* Bump supported Ubuntu version

NOTICE.txt

@@ -17,4 +17,4 @@ Swift Collections.
   * LICENSE (Apache License 2.0):
     * https://swift.org/LICENSE.txt
   * HOMEPAGE:
-    * https://github.com/apple/swift-collections
+    * https://github.com/apple/swift-collections

README.md

@@ -21,3 +21,117 @@
         <img src="https://img.shields.io/badge/twitter-codevapor-5AA9E7.svg" alt="Twitter">
     </a>
 </p>
+
+🏞 Multipart parser and serializer with `Codable` support for Multipart Form Data.
+
+### Major Releases
+
+The table below shows a list of MultipartKit major releases alongside their compatible NIO and Swift versions. 
+
+|Version|NIO|Swift|SPM|
+|---|---|---|---|
+|4.0|2.2|5.2+|`from: "4.0.0"`|
+|3.0|1.0|4.0+|`from: "3.0.0"`|
+|2.0|N/A|3.1+|`from: "2.0.0"`|
+|1.0|N/A|3.1+|`from: "1.0.0"`|
+
+Use the SPM string to easily include the dependency in your `Package.swift` file.
+
+```swift
+.package(url: "https://github.com/vapor/multipart-kit.git", from: ...)
+```
+
+### Supported Platforms
+
+MultipartKit supports the following platforms:
+
+- Ubuntu 18.04+
+- macOS 10.15+
+
+## Overview
+
+MultipartKit is a multipart parsing and serializing library. It provides `Codable` support for the special case of the `multipart/form-data` media type through a `FormDataEncoder` and `FormDataDecoder`. The parser delivers its output as it is parsed through callbacks suitable for streaming.
+
+### Multipart Form Data
+
+Let's define a `Codable` type and a choose a boundary used to separate the multipart parts.
+
+```swift
+struct User: Codable {
+    let name: String
+    let email: String
+}
+let user = User(name: "Ed", email: "ed@example.com")
+let boundary = "abc123"
+```
+
+We can encode this instance of a our type using a `FormDataEncoder`.
+
+```swift
+let encoded = try FormDataEncoder().encode(foo, boundary: boundary)
+```
+
+The output looks then looks like this.
+```
+--abc123
+Content-Disposition: form-data; name="name"
+
+Ed
+--abc123
+Content-Disposition: form-data; name="email"
+
+ed@example.com
+--abc123--
+```
+
+In order to _decode_ this message we feed this output and the same boundary to a `FormDataDecoder` and we get back an identical instance to the one we started with.
+
+```swift
+let decoded = try FormDataDecoder().decode(User.self, from: encoded, boundary: boundary)
+```
+
+### A note on `null`
+As there is no standard defined for how to represent `null` in Multipart (unlike, for instance, JSON), FormDataEncoder and FormDataDecoder do not support encoding or decoding `null` respectively. 
+
+### Nesting and Collections
+
+Nested structures can be represented by naming the parts such that they describe a path using square brackets to denote contained properties or elements in a collection. The following example shows what that looks like in practice.
+
+```swift
+struct Nested: Encodable {
+    let tag: String
+    let flag: Bool
+    let nested: [Nested]
+}
+let boundary = "abc123"
+let nested = Nested(tag: "a", flag: true, nested: [Nested(tag: "b", flag: false, nested: [])])
+let encoded = try FormDataEncoder().encode(nested, boundary: boundary)
+```
+
+This results in the content below.
+
+```
+--abc123
+Content-Disposition: form-data; name="tag"
+
+a
+--abc123
+Content-Disposition: form-data; name="flag"
+
+true
+--abc123
+Content-Disposition: form-data; name="nested[0][tag]"
+
+b
+--abc123
+Content-Disposition: form-data; name="nested[0][flag]"
+
+false
+--abc123--
+```
+
+Note that the array elements always include the index (as opposed to just `[]`) in order to support complex nesting.
+
+### Attribution
+
+This library contains code from the `OrderedCollection` module from https://github.com/apple/swift-collections. See: NOTICE.txt.