Create docs for extensions - Closes #1742

docs/EXTENSIONS_GUIDE.md

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+# Lisk Hub Extension Guideline
+``` ⚠️ Extension System is not avaliable on Production build so please use 'npm run dev' command ⚠️```
+
+## How to create a new extension?
+In your extension, you will have access to `React` and `LiskHubExtensions` that allows adding Your extension to specified areas in our code.
+
+![Alt text](./assets/create_extension_1.png?raw=true "Github File with extension")
+
+For building Your Component, You can use es6 and our wrapper will transpaile it for You.
+
+![Alt text](./assets/create_extension_2.png?raw=true "Github File with extension")
+
+#### LiskHubExtensions
+| Properties          |                    Description                 |
+| --------------- |---------------------------------------------|
+| `addModule({ identifier, component })` | identifier: `String`, component: `Component` <br/> Adds new component to the place we specify               |
+| `components`            | Contains shared styled components that We use in our app.             |
+| `identifiers`            | Identifier you pass to addModule() `dashboardColumn1, dashboardColumn2, dashboardColumn3`. It will add your component below dashboard components in specific column          |
+
+![Alt text](./assets/create_extension_3.png?raw=true "Github File with extension")
+
+#### Props
+Data from our store is accessible via `this.props.data` and actions to modify our store are in `this.props.actions`. Keep in mind that We are not sharing all data and actions. In case You want to access sth that is unavailable let us know by creating a feature proposal/ticket in our repo.
+```
+data = {
+  latestBlocks,
+  transactions,
+  time,
+  accountAddress,
+  search,
+}
+actions={
+  testExtensions,
+  loadTransactions,
+  sent,
+  loadTransaction,
+  transactionsFilterSet,
+  searchDelegate,
+  searchVotes,
+  searchVoters,
+  searchAccount,
+  searchTransactions,
+}
+```
+
+#### How to Share Your extension with others?
+Once Your component was created You need to add it our [repository](https://github.com/michaeltomasik/extensions-lisk). In order to do it 
+1. Create your own fork or branch.
+2. Add your component.
+3. Create Pull Request.
+
+First We need to verify Your code and then We will merge Your extension.
+
+In the future we will aim to create extension manager so users can easily add extension they want to their app.
+
+## How to add a new extension?
+
+1. Open our [repository](https://github.com/michaeltomasik/extensions-lisk) with all extensions.
+2. Click on the module you want to integrate.
+3. Once You have file open click `Raw` to get the code.
+
+![Alt text](./assets/add_extension_raw.png?raw=true "Github File with extension")
+
+4. Copy url link.
+
+![Alt text](./assets/add_extension_link.png?raw=true "Github raw link")
+
+5. Open `/extensions` page
+6. Paste Your url to input field and press Add Extension.
+
+![Alt text](./assets/add_extension_page.png?raw=true "Extension page")
+ The url is stored locally and The code from the repository will be transpiled, executed and added to the place you declared.
+
+7. Open page where You targeted Your component to be in lisk-hub.
+
+![Alt text](./assets/add_extension_dashboard.png?raw=true "Extension added to dashboard")
+
+