Add poser badges in README.md file

Author: ishan-biztech

README.md

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 # Xero OAuth2 API plugin for CakePHP 3.x
 
+[![Latest Stable Version](https://poser.pugx.org/ishan-biztech/cakephp-xero-oauth2/v)](//packagist.org/packages/ishan-biztech/cakephp-xero-oauth2)
+[![Total Downloads](https://poser.pugx.org/ishan-biztech/cakephp-xero-oauth2/downloads)](//packagist.org/packages/ishan-biztech/cakephp-xero-oauth2)
+[![License](https://poser.pugx.org/ishan-biztech/cakephp-xero-oauth2/license)](//packagist.org/packages/ishan-biztech/cakephp-xero-oauth2)
+[![Latest Unstable Version](https://poser.pugx.org/ishan-biztech/cakephp-xero-oauth2/v/unstable)](//packagist.org/packages/ishan-biztech/cakephp-xero-oauth2)
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 This plugin provides access to Xero OAuth2 API. This plugin is wrapper around [Xero PHP official SDK](https://github.com/XeroAPI/xero-php-oauth2/).
 
 ## Requirements