Added the section 'Configuration' in /docs/turn-howto.md (#6614)

put the 2nd part of the "source installation"-section into a new section, because it also applies to Debian packages

changelog.d/6614.doc

@@ -0,0 +1 @@
+Added the section 'Configuration' in /docs/turn-howto.md.

docs/turn-howto.md

@@ -39,6 +39,8 @@ The TURN daemon `coturn` is available from a variety of sources such as native p
         make
         make install
 
+### Configuration
+
 1.  Create or edit the config file in `/etc/turnserver.conf`. The relevant
     lines, with example values, are: