[DOCS] Adds section about PSR 7.

Author: szabosteve

docs/overview.asciidoc

@@ -12,4 +12,13 @@ from one language to the next with minimal effort.
 The client is designed to be "unopinionated". There are a few universal niceties 
 added to the client (cluster state sniffing, round-robin requests, and so on) 
 but largely it is very barebones. This was intentional; we want a common base 
-that more sophisticated libraries can build on top of.
+that more sophisticated libraries can build on top of.
+
+[discrete]
+[[psr-7-standard]]
+=== PSR 7 standard
+
+The {es} PHP client uses the https://www.php-fig.org/psr/[PSR] 7 standard. This 
+standard is a community effort that contains a set of interfaces defined by the 
+PHP Framework Interop Group. For more information, refer to the 
+https://www.php-fig.org/psr/psr-7/[PSR 7 standard documentation].