Merge pull request #1137 from szabosteve/php.standard

[DOCS] Adds section about PSR 7

Author: szabosteve

docs/overview.asciidoc

@@ -14,11 +14,22 @@ 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.
 
+
+[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].
+
+
 * <<community_dsls>>
 * <<community-integrations>>
 * <<breaking_changes>>
 
 
 include::community.asciidoc[]
 
-include::breaking-changes.asciidoc[]
+include::breaking-changes.asciidoc[]