docs: setting environment variables of an app happens before cloning template code

[zimeg]

📚 The current README sections suggest updating the ".env" file with app credentials then cloning this template, but we might want to prefer the opposite order?

bolt-js-assistant-template/README.md

Lines 49 to 86 in 65f5ab0

	### Using Terminal
	1. Open [https://api.slack.com/apps/new](https://api.slack.com/apps/new) and choose "From an app manifest"
	2. Choose the workspace you want to install the application to
	3. Copy the contents of [manifest.json](./manifest.json) into the text box that says `*Paste your manifest code here*` (within the JSON tab) and click _Next_
	4. Review the configuration and click _Create_
	5. Click _Install to Workspace_ and _Allow_ on the screen that follows. You'll then be redirected to the App Configuration dashboard.
	#### Environment Variables
	Before you can run the app, you'll need to store some environment variables.
	1. Rename `.env.sample` to `.env`.
	2. Open your apps setting page from [this list](https://api.slack.com/apps), click _OAuth & Permissions_ in the left hand menu, then copy the _Bot User OAuth Token_ into your `.env` file under `SLACK_BOT_TOKEN`.
	```sh
	SLACK_BOT_TOKEN=YOUR_SLACK_BOT_TOKEN
	```
	3. Click _Basic Information_ from the left hand menu and follow the steps in the _App-Level Tokens_ section to create an app-level token with the `connections:write` scope. Copy that token into your `.env` as `SLACK_APP_TOKEN`.
	```sh
	SLACK_APP_TOKEN=YOUR_SLACK_APP_TOKEN
	```
	#### Initializing the project
	```sh
	git clone https://github.com/slack-samples/bolt-js-assistant-template.git my-bolt-js-assistant
	cd my-bolt-js-assistant
	```
	#### Install dependencies
	```sh
	npm install
	```