MT#55897 Introduce 'assets' folder for repository resources

We need to introduce a dedicated folder for repo resources,
such as images used in the wiki page.

Additionally: add right away first dias for the
"RTPEngine project architecture" wiki page.

Change-Id: Ie2407043dbc215f0db22c5cec46cb3ee2cbe986b

assets/README.md

@@ -0,0 +1,10 @@
+rtpengine assets
+==================================
+
+Designation of this folder
+--------------------
+
+This is a folder for the repository related resources.
+It's not dedicated to store parts of the project itself, such as code.
+
+An example of repository needs could be images (diagrams) used in the wiki.