interactive bots: Create Howdoi bot.

This bot allows users to search technical questions from Stack
Overflow.

contrib_bots/lib/HowdoiBot/docs.md

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+# Howdoi bot
+
+This bot will allow users to get technical answers from
+[StackOverflow](https://stackoverflow.com). It is build on top of the
+python command line tool [howdoi](https://github.com/gleitz/howdoi) by
+Benjamin Gleitzman.
+
+## Usage
+
+Simply prepend your questions with one of the following commands. The
+answer will be formatted differently depending the chosen command.
+
+| Command     | Respond                                                |
+| ----------- | ------------------------------------------------------ |
+| `@howdowe`  | Concise answer to the same stream.                     |
+| `@howdowe!` | Same as `@howdowe` but with full answer and URL of the solutions. |
+| `@howdoi`   | Concise answer replied to sender via private message.  |
+| `@howdoi!`  | Same as `@howdoi` but with full answer and URL of the solutions. |
+
+## Screenshots
+
+#### Example 1
+
+Question -> `@howdowe use supervisor in elixir`
+
+  ![howdowe question](question_howdowe.png)
+
+Answer ->  Howdoi would try to **only** respond with the coding section
+of the answer.
+
+  ![howdowe answer](answer_howdowe.png)
+
+#### Example 2
+
+Question -> `@howdoi! stack vs heap`
+
+  ![howdoi! question](question_howdoi_all.png)
+
+Answer -> Howdoi would return the **full** stackoverflow answer via
+**private message** to the original sender. The URL of the answer can be
+seen at the bottom of the message.
+
+  ![howdoi! answer](answer_howdoi_all.png)
+
+**Note:**
+
+* Line wrapped is enabled with a maximum line length of 85 characters.
+This could be adjusted in the source code (`HowdoiHandler.MAX_LINE_LENGTH`).
+
+* *Howdoi* generally perform better if you ask a question using keywords
+instead of a complete sentences (eg: "How do i make a decorator in Python"
+-> "python decorator").
+
+* __[*Limitation*]__ If a answer contains multiple code blocks, the `@howdoi`
+and `@howdowe` commands would only return the first coding section, use
+`@howdo[we|i]!` in that case.