Added note about current bug with autoload (#178)

* Added note about current bug with autoload

I added a note + temporary workaround for deployphp/deployer#1371 so that users don't have to waste time figuring out why this isn't working currently. This will be reverted hopefully once v7 is released with a fix.
I also added better examples to the configurations so that new users would know exactly how to use the commands.

* Fixed typo

removed extra 'required' which was a typo.

docs/slack.md

@@ -4,10 +4,17 @@
 
 <a href="https://slack.com/oauth/authorize?&client_id=113734341365.225973502034&scope=incoming-webhook"><img alt="Add to Slack" height="40" width="139" src="https://platform.slack-edge.com/img/add_to_slack.png" srcset="https://platform.slack-edge.com/img/add_to_slack.png 1x, https://platform.slack-edge.com/img/add_to_slack@2x.png 2x" /></a>
 
-Require slack recipe in your `deploy.php` file:
+Install recipes through composer:  
+```
+composer require deployer/recipes --dev
+```
+
+Require slack recipe in your `deploy.php` file:  
+**note:** currently there is a bug in the autoloader detection requiring users to specify the full path ([more info](https://github.com/deployphp/deployer/issues/1371)).
 
 ```php
-require 'recipe/slack.php';
+//require 'recipe/slack.php';
+require 'vendor/deployer/recipe/recipes/slack.php';
 ```
 
 Add hook on deploy:
@@ -19,18 +26,21 @@ before('deploy', 'slack:notify');
 ## Configuration
 
 - `slack_webhook` – slack incoming webhook url, **required** 
+  ```
+  set('slack_webhook', 'https://hooks.slack.com/...');
+  ```
 - `slack_title` – the title of application, default `{{application}}`
 - `slack_text` – notification message template, markdown supported
   ```
-  _{{user}}_ deploying `{{branch}}` to *{{target}}*
+  set('slack_text', '_{{user}}_ deploying `{{branch}}` to *{{target}}*');
   ```
 - `slack_success_text` – success template, default:
   ```
-  Deploy to *{{target}}* successful
+  set('slack_success_text', 'Deploy to *{{target}}* successful');
   ```
 - `slack_failure_text` – failure template, default:
   ```
-  Deploy to *{{target}}* failed
+  set('slack_failure_text', 'Deploy to *{{target}}* failed');
   ```
 
 - `slack_color` – color's attachment