Add .readthedocs.yml

readthedocs.org allows to automatically generate and publish the HTML
documentation for the U-Boot project.

Add a file controlling building https://u-boot.readthedocs.io/.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

.readthedocs.yml

@@ -0,0 +1,19 @@
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Optionally build your docs in additional formats such as PDF and ePub
+formats: []
+
+# Optionally set the version of Python and requirements required to build your docs
+# python:
+#   version: 3.7
+#   install:
+#     - requirements: docs/requirements.txt