add build status badge to README

Links are relative to the current repo, so when forking the repo,
the build status refers to the fork rather than the parent repo.

Author: wnienhaus

README.rst

@@ -2,6 +2,11 @@
 py-esp32-ulp
 =====================
 
+.. image:: ../../actions/workflows/run_tests.yaml/badge.svg
+   :height: 20px
+   :target: ../../actions/workflows/run_tests.yaml
+   :alt: Build Status
+
 py-esp32-ulp is an assembler toolchain for the ESP32 ULP (Ultra Low-Power)
 Co-Processor, written in MicroPython.