Add documents for GitLab utilitiesdocs-gitlab-utilities

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diff --git a/doc/development/README.md b/doc/development/README.md
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@@ -27,6 +27,7 @@ comments: false
 
 ## Backend guides
 
+- [GitLab utilities](utilities.md)
 - [API styleguide](api_styleguide.md) Use this styleguide if you are
   contributing to the API.
 - [Sidekiq guidelines](sidekiq_style_guide.md) for working with Sidekiq workers
diff --git a/doc/development/utilities.md b/doc/development/utilities.md
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+# GitLab utilities
+
+We developed a number of utilities to ease development.
+
+## [`MergeHash`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/utils/merge_hash.rb)
+
+* Deep merges an array of hashes:
+
+    ``` ruby
+    Gitlab::Utils::MergeHash.merge(
+      [{ hello: ["world"] },
+       { hello: "Everyone" },
+       { hello: { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] } },
+        "Goodbye", "Hallo"]
+    )
+    ```
+
+    Gives:
+
+    ``` ruby
+    [
+      {
+        hello:
+          [
+            "world",
+            "Everyone",
+            { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] }
+          ]
+      },
+      "Goodbye"
+    ]
+    ```
+
+* Extracts all keys and values from a hash into an array:
+
+    ``` ruby
+    Gitlab::Utils::MergeHash.crush(
+      { hello: "world", this: { crushes: ["an entire", "hash"] } }
+    )
+    ```
+
+    Gives:
+
+    ``` ruby
+    [:hello, "world", :this, :crushes, "an entire", "hash"]
+    ```
+
+## [`StrongMemoize`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/utils/strong_memoize.rb)
+
+* Memoize the value even if it is `nil` or `false`.
+
+    We often do `@value ||= compute`, however this doesn't work well if
+    `compute` might eventually give `nil` and we don't want to compute again.
+    Instead we could use `defined?` to check if the value is set or not.
+    However it's tedious to write such pattern, and `StrongMemoize` would
+    help us use such pattern.
+
+    Instead of writing patterns like this:
+
+    ``` ruby
+    class Find
+      def result
+        return @result if defined?(@result)
+
+        @result = search
+      end
+    end
+    ```
+
+    We could write it like:
+
+    ``` ruby
+    class Find
+      include Gitlab::Utils::StrongMemoize
+
+      def result
+        strong_memoize(:result) do
+          search
+        end
+      end
+    end
+    ```
+
+* Clear memoization
+
+    ``` ruby
+    class Find
+      include Gitlab::Utils::StrongMemoize
+    end
+
+    Find.new.clear_memoization(:result)
+    ```