README: Add GH Actions badges
Makes it easier to catch broken CI on master branch.
## Overview
+<a href="https://github.com/apache/mynewt-nimble/actions/workflows/build_targets.yml">
+ <img src="https://github.com/apache/mynewt-nimble/actions/workflows/build_targets.yml/badge.svg">
+<a/>
+
+<a href="https://github.com/apache/mynewt-nimble/actions/workflows/build_ports.yml">
+ <img src="https://github.com/apache/mynewt-nimble/actions/workflows/build_ports.yml/badge.svg">
+<a/>
+
+<a href="https://github.com/apache/mynewt-nimble/actions/workflows/newt_test_all.yml/badge.svg">
+ <img src="https://github.com/apache/mynewt-nimble/actions/workflows/newt_test_all.yml/badge.svg">
+<a/>
+
+<p>
+
Apache NimBLE is an open-source Bluetooth 5.1 stack (both Host & Controller)
that completely replaces the proprietary SoftDevice on Nordic chipsets. It is
part of [Apache Mynewt project](https://github.com/apache/mynewt-core).
