readme: add missing logo

Logo was accidentally deleted as part of some docs cleanup. Add it back
with a new name making it more explicit that it belongs to the readme.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

README.rst

@@ -2,7 +2,7 @@
 
    <a href="https://www.zephyrproject.org">
      <p align="center">
-       <img src="doc/images/Zephyr-Project.png">
+       <img src="doc/_static/images/logo-readme.png">
      </p>
    </a>