Create a small guide for Collections

SUMMARY.md

@@ -137,5 +137,6 @@
 * [Guides](guides/README.md)
    * [Performance](guides/performance.md)
    * [Concurrency](guides/concurrency.md)
+   * [Collections] (guides/collections.md)
    * [Testing](guides/testing.md)
    * [Writing Shards](guides/writing_shards.md)

guides/collections.md

@@ -0,0 +1,27 @@
+# Collections
+
+Collections of objects, such as [Array](https://crystal-lang.org/api/Array.html), [Hash](https://crystal-lang.org/api/Hash.html), and [Set](https://crystal-lang.org/api/Set.html) are powerful data structures for storing groups of data. Crystal offers a number of Modules to assist developers working with collections of objects in the Standard Library. 
+
+
+```
+arr = Array(String).new
+arr << "Foo"
+arr << "Bar"
+arr << "Baz"
+arr << "Foo"
+
+arr.size # returns 4
+arr.empty? # returns false
+
+arr.count "Foo" # returns 2
+arr.count "Nope" # returns 0
+
+arr.index { |i| i == "Baz" } # returns 1
+arr.index { |i| i == "Nope" } # returns 
+
+arr.find { |i| i == "Foo" } # return "Foo"
+arr.find { |i| i == "Nope" } # return nil
+
+```
+
+For full details, see the [Indexable](https://crystal-lang.org/api/Indexable.html), [Enumerable]((https://crystal-lang.org/api/Enumerable.html), [Iterable](https://crystal-lang.org/api/Iterable.html) Modules.