Use https:// for URLs in documents

Also adds contribution guidelines (for Github) with links to the wiki.

NEWS

@@ -1081,7 +1081,7 @@ strongswan-5.0.3
   charon-tkm does not result in the compromise of cryptographic keys.
   The extracted functionality has been implemented from scratch in a minimal TCB
   (trusted computing base) in the Ada programming language. Further information
-  can be found at http://www.codelabs.ch/tkm/.
+  can be found at https://www.codelabs.ch/tkm/.
 
 strongswan-5.0.2
 ----------------
@@ -1219,7 +1219,7 @@ strongswan-5.0.0
   pluto, but currently does not support AH or bundled AH+ESP SAs. Beside
   RSA/ECDSA, PSK and XAuth, charon also supports the Hybrid authentication
   mode. Information for interoperability and migration is available at
-  http://wiki.strongswan.org/projects/strongswan/wiki/CharonPlutoIKEv1.
+  https://wiki.strongswan.org/projects/strongswan/wiki/CharonPlutoIKEv1.
 
 - Charon's bus_t has been refactored so that loggers and other listeners are
   now handled separately.  The single lock was previously cause for deadlocks
@@ -1650,7 +1650,7 @@ strongswan-4.4.0
 - The IKEv2 High Availability plugin has been integrated. It provides
   load sharing and failover capabilities in a cluster of currently two nodes,
   based on an extend ClusterIP kernel module. More information is available at
-  http://wiki.strongswan.org/projects/strongswan/wiki/HighAvailability.
+  https://wiki.strongswan.org/projects/strongswan/wiki/HighAvailability.
   The development of the High Availability functionality was sponsored by
   secunet Security Networks AG.
 
@@ -2358,7 +2358,7 @@ strongswan-4.1.7
 
 - Preview of strongSwan Manager, a web based configuration and monitoring
   application. It uses a new XML control interface to query the IKEv2 daemon
-  (see http://wiki.strongswan.org/wiki/Manager).
+  (see https://wiki.strongswan.org/wiki/Manager).
 
 - Experimental SQLite configuration backend which will provide the configuration
   interface for strongSwan Manager in future releases.