Initial ADR Readme

Added Architectural Decision Record (ADR) naming convention.

Signed-off-by: Robert Drab <robert.drab@huawei.com>

adr/README.md

+Architectural Decision Records
+==============================
+
+Here we keep all Architectural Decision Records.
+Each decision shall be in a separate file with name following convention:
+
+NNNN-title-with-dashes.md
+
+where the NNNN is a decision number.