Added step-by-step Heroku deployment guide

Author: darrenvong

assets/create_heroku_app.png

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+# Deploying your app to Heroku - a step by step guide
+
+Up until now, we have been working on our app and running it locally on our laptops. Whilst
+this is useful for testing out our app during development, it's no good if our friends want to see
+what we've done in this course!
+
+In order for our friends to see the app, we need to upload (deploy) our code to Heroku, a cloud-based platform
+which will assign a publicly accessible URL to our app that can then be visited!
+
+Without further ado, let's go through how you can deploy your first app to Heroku!
+
+0. Before you start to deploy your app, there are some chores we need to do first:
+   * Make a `Procfile` in the **root folder** of your application. This file contains the Command Prompt/Terminal command telling Heroku exactly how to start/run your application.
+   It normally contains only the following line:
+   ```
+   web: python app.py
+   ```
+   Replace `app.py` with the actual name of your file containing your Flask code if you named it something different.
+   In case you're curious: the `web:` part before the command basically tells Heroku to run your Python code as a web application.
+   
+   * Make a file named `requirements.txt` in the **root folder** of your application. This file lists all the external libraries
+   (such as `Flask` and `tweepy`) required by your application, so that Heroku knows exactly the required libraries to install
+   before trying to run your application.
+   In this file, type in the libraries your application require line by line, which may look something like the following:
+   ```
+   flask
+   requests
+   tweepy
+   requests
+   ```
+   
+   * In `app.py` (or whatever you named your main file containing the Flask code), add the following line to the top
+   of your Flask code:
+   ```python
+   import os
+   ```
+   For the curious: the `os` module is imported so that we can access *environment variables* set by Heroku, which includes the
+   *port* it has assigned to run your application on.
+   
+   Next (usually near the bottom of your Flask code), replace
+   ```python
+   app.run(debug=True)
+   ```
+   with the following:
+   ```python
+   if 'PORT' in os.environ:
+        app.run(host='0.0.0.0', port=int(os.environ['PORT']))
+   else:
+        app.run(debug=True)
+   ```
+   Again, for the curious: this piece of conditional logic checks whether you are running your application on Heroku
+   (in which case, the `PORT` environment variable would exist), or if you are running your application locally. The
+   `host=0.0.0.0` bit instructs your Flask application to listen on **all** web address. This is important to ensure
+   the application runs on Heroku since we don't know which host Heroku will decide to host the application on.
+
+1. If you haven't already, go make a Heroku account at https://www.heroku.com
+
+2. Once you have made a Heroku account, upon logging in, you will be taken to a dashboard, which looks something like the
+following:
+
+![alt text](assets/heroku_dashboard.png "Heroku Dashboard")
+
+3. To deploy the app, we need to make one first on Heroku, so do that by clicking **New > Create New app** (button at top right of the
+dashboard)
+
+4. Type in the name of your application, and change the Runtime Selection to "Europe" - this is so that the application loads
+faster as we are geographically closer to Europe than the United States. Finally, click "Create App".
+
+![alt text](assets/create_heroku_app.png "Create app screen")
+
+5. After clicking "Create App", you should be greeted with the following screen:
+
+![alt text](assets/heroku_config.png "App configuration screen")
+
+6. Under "Deployment Method", change it to "GitHub". A "Connect to GitHub" section should now appear.
+
+7. Under the "Connect to GitHub" section that just appeared, search for the GitHub repository corresponding to your project, then
+click "Connect" to link the relevant repository (appeared as a result of the search) to the Heroku application you've just created.
+
+8. To deploy your project, click "Deploy Branch" under "Manual deploy".
+
+9. (Optional) You may have just noticed that a section named "Automatic deploys" has appeared in addition to the "Manual deploy"
+section once you have connected your GitHub repository properly. To save you having to come back to this page in the future
+and click "Deploy Branch" each time you pushed something new to your repository, if you click "Enable Automatic Deploys", then
+Heroku will do so automatically.
+
+Congratulations! You have now successfully deployed your application to Heroku!