Merge pull request #3870 from shaedrich/psr-15

Add topic "PSR 15"

topics/psr-15/index.md

@@ -0,0 +1,9 @@
+---
+display_name: "PSR-15: HTTP Server Request Handlers"
+short_description: PSR-7 is a set of common interfaces representing HTTP server request handlers, and HTTP server middleware components.
+topic: psr-15
+related: psr, psr-7
+url: https://www.php-fig.org/psr/psr-15/
+created_by: Woody Gilk
+---
+**PSR 15** is a set of common interfaces for HTTP server request handlers ("request handlers") and HTTP server middleware components ("middleware") that use HTTP messages as described by PSR-7 or subsequent replacement PSRs.