Created Contributing Guide, added some info to it

CONTRIBUTING.md

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+# Contributions
+
+## Set up project for development
+1. Fork this repository by clikcing the **Fork** button.
+1. `git clone https://github.com/<your username>/componofy.git`
+1. Set upstream: `git remote add upstream https://github.com/DalerAsrorov/componofy.git`
+1. Create a new branch `git checkout -b [feature branch name]`
+1. `cd ./componofy`
+1. `yarn install`
+1. In [`src/utils/helpers.js`](src/utils/helpers.js) replace
+[`DEV_SERVER_URL`](src/utils/helpers.js#L5) with `HOST_URL` in [line 8](src/utils/helpers.js#L8)
+1. Create an `.env` file with the following information:
+```
+APP_CLIENT_URL=http://localhost:3000
+SPOTIFY_CLIENT=[your own spotify client key]
+SPOTIFY_CLIENT_SECRET=[your own spotify client secret key]
+SPOTIFY_REDIRECT_URI=http://localhost:3001/api/callback
+YAR_PASS=[any set of characters (without quotes)]
+DEV_MODE=true
+```
+9. `yarn start:api` in one terminal (window/tab)
+1. `yarn start:dev` in another terminal (window/tab)
+
+The dev server will be started and the browser with the app will be opened.
+The project should be available in `localhost:3000`.
+Note: you can get spotify client app and secret keys in 
+[Developer's](https://developer.spotify.com/) website section
+
+## Building a new feature
+Depending on the type of the feature, there are different places in the project where the changes can be made.
+
+### Adding new component
+1. Create a new folder with the component name in [`./src/components`](src/components).
+The name of the folder should be capitalized and camel case.
+1. Add two new files `index.js` and `[NameOfTheComponentFolder].scss`. 
+Save the `.scss` file. `yarn start:dev` script will generate `[NameOfTheComponentFolder].css` file which you can
+import in `index.js` like 
+```
+import './[NameOfTheComponentFolder].css'
+```
+1. To keep the look and feel of the app consistent, use Material UI react framework 
+that is used and is encouraged.
+1. See [`src/components/FooterPanel`](src/components/FooterPanel) as an example.
+1. Once the component is implemented, you will use it in one of the base components:
+  - [`src/components/MyPlaylists/index.js`](src/components/MyPlaylists/index.js)
+  - [`src/components/PublicPlalists/index.js`](src/components/PublicPlaylists/index.js)
+  - [`src/components/ComponofyPlaylists/index.js`](src/components/ComponofyPlaylists/index.js)
+* If component should be used in the dialog window (final step of the app)
+  - [`src/components/Dialog/index.js`](src/components/Dialog/index.js)
+
+### Adding new action/reducer/prop
+ TODO
+
+### Adding new endpoint
+ TODO
+
+## Before submitting new PR
+1. `git checkout master`
+1. `git pull --rebase upstream master`
+1. `git checkout [your feature branch name]`
+1. In [`src/utils/helpers.js`](src/utils/helpers.js#L5) place back
+[`HOST_URL`](src/utils/helpers.js#L3) in [line 8](src/utils/helpers.js#L8)
+1. Commit this change to new branch
+1. Create a PR with your new branch
+
+
+