Create Suffix.md (#82)

Guide for suffix(while: ) function

Author: dhruvshah8

Guides/Suffix.md

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+# Suffix
+
+[[Source](https://github.com/apple/swift-algorithms/blob/main/Sources/Algorithms/Suffix.swift) |
+ [Tests](https://github.com/apple/swift-algorithms/blob/main/Tests/SwiftAlgorithmsTests/SuffixTests.swift)]
+
+
+This function returns a subsequence containing the elements from the end of the collection until predicate returns `false` and skipping the remaining elements.
+
+This example uses `suffix(while: )` to iterate through collection of integers from the end until the predicate returns false, in this case when `$0 <= 5`
+```swift
+(0...10).suffix(while: { $0 > 5 } // == [6,7,8,9,10]
+```
+
+
+## Detailed Design
+
+The `suffix(while:)` function is added as a method on an extension of  `BidirectionalCollection`.
+
+
+```swift
+extension BidirectionalCollection {
+
+  public func suffix(while predicate: (Element) throws -> Bool) rethrows -> SubSequence
+}
+```
+
+This method requires `BidirectionalCollection` for an efficient implementation which visits as few elements as possible. Swift's protocol allows for backward traversal of a collection as well as access to *last* property of a collection.
+
+### Complexity
+
+Calling this method is O(*n*), where *n* is the length of the collection.
+
+
+### Naming
+
+The function's name resembles that of an existing Swift function `prefix(while:)`, which performs same operation however in the forward direction of the collection. Hence, as this function traverses from the end of the collection, `suffix(while:)` is an appropriate name.
+
+
+