Add an example of a default @context

defaultContext/defaultContext.jsonld

+{
+  "@context": [
+    "https://fiware.github.io/NGSI-LD_Tests/ldContext/defaultContextVocab.jsonld",
+    "https://forge.etsi.org/gitlab/NGSI-LD/NGSI-LD/raw/master/coreContext/ngsi-ld-core-context.json"
+  ]
+}

defaultContext/defaultContextVocab.jsonld

+{
+  "@context": {
+    "@vocab": "http://example.org/ngsi-ld/default/"
+  }
+}