Add a short README for the nginx container

Explain why we provide an own nginx container image.

Author: bjoernricks

README.md

@@ -12,3 +12,4 @@ For example `docker pull registry.community.greenbone.net/community/gpg-data`
 For more details please take a look at
 
 - [gvm-config](./gvm-config/README.md)
+- [nginx](./nginx/README.md)

nginx/README.md

@@ -0,0 +1,8 @@
+# nginx Container Image
+
+A nginx container image based on `nginx:stable` to support auto reloading of
+config file changes.
+
+Because the [gvm-config](../gvm-config/README.md) container image generates a
+nginx configuration on each startup, nginx needs to be restarted to load the
+new and possibly changed configuration file.