Modify the adjacent pairs guide into a proposal ready for review (#178)

* Modify the adjacent pairs guide into a proposal ready for review

* Update the proposal # to 0005

Author: phausler

Diff for: Evolution/0005-adjacent-pairs.md

@@ -0,0 +1,55 @@
+# AdjacentPairs
+
+* Proposal: [SAA-0005](https://github.com/apple/swift-async-algorithms/blob/main/Evolution/0005-adjacent-pairs.md)
+* Author(s): [László Teveli](https://github.com/tevelee)
+* Review Manager: [Philippe Hausler](https://github.com/phausler)
+* Status: **Implemented**
+* Implementation: [[Source](https://github.com/apple/swift-async-algorithms/blob/main/Sources/AsyncAlgorithms/AsyncAdjacentPairsSequence.swift) | 
+ [Tests](https://github.com/apple/swift-async-algorithms/blob/main/Tests/AsyncAlgorithmsTests/TestAdjacentPairs.swift)]
+* Decision Notes: 
+* Bugs: 
+
+## Introduction
+
+The `adjacentPairs()` API serve the purpose of collecting adjacent values. This operation is available for any `AsyncSequence` by calling the `adjacentPairs()` method.
+
+```swift
+extension AsyncSequence {
+  public func adjacentPairs() -> AsyncAdjacentPairsSequence<Self>
+}
+```
+
+## Detailed Design
+
+The `adjacentPairs()` algorithm produces elements of tuple (size of 2), containing a pair of the original `Element` type. 
+
+The interface for this algorithm is available on all `AsyncSequence` types. The returned `AsyncAdjacentPairsSequence` conditionally conforms to `Sendable`.
+
+Its iterator keeps track of the previous element returned in the `next()` function and updates it in every turn.
+
+```swift
+for await (first, second) in (1...5).async.adjacentPairs() {
+   print("First: \(first), Second: \(second)")
+}
+
+// First: 1, Second: 2
+// First: 2, Second: 3
+// First: 3, Second: 4
+// First: 4, Second: 5
+```
+
+It composes well with the [Dictionary.init(_:uniquingKeysWith:)](https://github.com/apple/swift-async-algorithms/blob/main/Guides/Collections.md) API that deals with `AsyncSequence` of tuples.
+
+```swift
+Dictionary(uniqueKeysWithValues: url.lines.adjacentPairs())
+```
+
+## Alternatives Considered
+
+This functionality is often written as a `zip` of a sequence together with itself, dropping its first element (`zip(source, source.dropFirst())`).
+
+It's such a dominant use-case, the [swift-algorithms](https://github.com/apple/swift-algorithms) package also [introduced](https://github.com/apple/swift-algorithms/pull/119) it to its collection of algorithms.
+
+## Credits/Inspiration
+
+The synchronous counterpart in [swift-algorithms](https://github.com/apple/swift-algorithms/blob/main/Guides/AdjacentPairs.md).