created dev setup guide

Author: PascalMayr

Diff for: README.md

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+# InstaShare
+
+A simple application to upload large files and share them instantly.
+
+# Dev setup guide
+
+  - Download and install MongoDB on your system: https://docs.mongodb.com/manual/installation/
+  - Start the Mongo deamon
+  - Clone and cd into the repo
+  
+  SERVER SETUP:
+  
+  - Go into the server folder
+  - Create a .env file at the root specifying:
+     ```
+     JWT_SECRET=<SET_RANDOM_SECRET_KEY/> - use 'openssl rand -base64 32' to create a secret key
+     DB_CONNECTION=<SET_DB_CONNECTION/> - on localhost mostly: mongodb://localhost:27017/
+     DB_NAME=<SET_DB_NAME/>  - the name of a new or already existing db
+     BUCKET_NAME=<SET_BUCKET_NAME/>  - the name of the  bucket where your files will be stored
+     PORT=<SET_SERVER_PORT/> - the port on which your express server will be started. Default is 5000
+     ```
+  - install node_modules with yarn install
+     ```sh
+    $ yarn install
+    ```
+  - install nodemon globally: https://www.npmjs.com/package/nodemon
+  - start the server
+    ```sh
+    $ yarn run dev
+    ```
+    
+  CLIENT SETUP:
+  - Go into the client folder
+  - Create a .env file at the root specifying:
+      ```
+     REACT_APP_SERVER_HOST=<SERVER_HOST /> - default should be: http://localhost:5000/
+     ```
+  - install node_modules with yarn install
+     ```sh
+    $ yarn install
+    ```
+  - start the react development server
+    ```sh
+    $ yarn run start
+    ```
+    You can now sign up and upload your first files!