[Readme] Add instructions for multiple applications on same domain (p…

…imcore#17699)

* add instrcutions for multiple applications on same domain

* Create 08_Multi_Application_Setup.md

* Update 08_Multi_Application_Setup.md

* Update 08_Multi_Application_Setup.md

doc/23_Installation_and_Upgrade/03_System_Setup_and_Hosting/08_Multi_Application_Setup.md

@@ -0,0 +1,23 @@
+# Multi-application setup
+
+## Sessions
+
+When running multiple applications on the same domain, there can be session cookie collisions which prevent you to log in to both systems at the same time.
+
+Imagine you run a web shop app on http://example.org and Pimcore on http://pim.example.org. Then you will have 2 cookies with name `PHPSESSID` (if `session.name` in php.ini is the same for both):
+| Name| Value  | Domain  |  Path | 
+|---|---|---|---|
+| PHPSESSID | 5a9b08750387d9e11c738a2947d93e38   |  .example.org | /  | 
+| PHPSESSID | irqnjh5p96gp2i8iu743ulm32p   |  pim.example.org | /  | 
+
+First one is from the web shop, second one from Pimcore.
+When trying to log in at http://example.org/admin you will get a 403 Forbidden error.
+(The reason why the web shop sets the cookie for `.example.org` instead of `example.org` probably is to also support subdomains.)
+
+You can prevent this problem with the following config in your `config.yaml`. This way the session cookies do not conflict anymore and you will be able to log in to both applications at the same time.
+```yaml
+framework:
+    session:
+        name: "PIMCORE_SESSION_ID"
+```
+

doc/23_Installation_and_Upgrade/03_System_Setup_and_Hosting/README.md

@@ -9,4 +9,5 @@
 
 ## Advanced Topics
 - [Cluster Setup](07_Cluster_Setup.md)
+- [Multi-application setup](08_Multi_Application_Setup.md)
 - [Performance Guide](09_Performance_Guide.md)