docs(README.md): Update Readme.md (#443)

* chore(release): 3.23.2

* Update Readme.md

added link to signup page under installation.

Co-authored-by: pcholuj <piotr.choluj@filestack.com>

Author: KenichiRuf

README.md

@@ -64,6 +64,9 @@
 npm install filestack-js
 ```
 
+To get your free API key, sign up for a Filestack account here:
+[https://dev.filestack.com/signup/free](https://dev.filestack.com/signup/free).
+
 ## API Documentation
 
 [https://filestack.github.io/filestack-js/](https://filestack.github.io/filestack-js/)