docs: add contributing file (#18)

CONTRIBUTING.md

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+# How to contribute to the project
+
+This project welcomes any change, improvement, or suggestion!
+
+If you'd like to help its development feel free to open a new issue and raise a pull request.
+
+## IMPORTANT
+
+If you'd like to work on an existing issue, kindly **ask** for it to be assigned to you.
+
+Do you have any struggles with the issue you are working on? Feel free to **tag [me](https://github.com/Romaixn)** in it _and/or_ open a draft pull request.
+
+
+### How do I make a contribution
+
+If you've never made an open source contribution before or are curious about how contributions operate in our project? Here's a quick rundown!
+
+#### Fork this repository
+
+Fork this repository by clicking on the fork button on the top of [this](https://github.com/Romaixn/gosumer) page.
+This will create a copy of this repository in your account `<your-GitHub-username>/<repository-name>`.
+
+#### Clone the repository
+
+Now clone the forked repository to your machine. Go to your GitHub account, open the forked repository, and copy the link provided under `HTTPS` when you click on the green button labeled `code` on the repository page
+
+Open a terminal and run the following git command:
+
+```
+git clone "url you just copied"
+```
+
+where "URL you just copied" (without quotation marks) is the URL to this repository (your fork of this project). 
+
+For example:
+
+```
+git clone https://github.com/username/gosumer.git
+```
+
+#### Create a new branch for your changes or fix 
+
+    ```sh
+     $ git checkout -b <branch-name>
+    ```
+
+#### Setup the project in your local by following the steps listed in the [README.md](https://github.com/Romaixn/gosumer/blob/main/README.md) file
+
+#### Open the project in a code editor and begin working on it
+#### Add the contents of the changed files to the "snapshot" git uses to manage the state of the project, also known as the index
+
+    ```sh
+    $ git add .
+    ```
+
+#### Add a descriptive commit message
+
+    ```sh
+    $ git commit -m "Insert a short message of the changes made here"
+    ```
+
+#### Push the changes to the remote repository
+
+    ```sh
+    $ git push -u origin <branch-name>
+    ```
+
+#### Submit a pull request to the upstream repository