Update README as markdown

Author: taranu

README

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+# lsst-sconsUtils
+
+[![codecov](https://codecov.io/gh/lsst/sconsUtils/branch/main/graph/badge.svg?token=2BUBL8R9RH)](https://codecov.io/gh/lsst/sconsUtils)
+
+sconsUtils is a package in the [LSST Science Pipelines](https://pipelines.lsst.io/).
+
+This package contains utility scripts for building pipelines packages with 
+[SCons](https://scons.org/), and is required to build most pipelines packages.
+SCons can be used to build C++ and Python (with pybind11 bindings) packages;
+see the [stack package templates](https://github.com/lsst/templates/tree/main/project_templates/stack_package)
+if this is of interest.
+
+This is a **Python 3 only** package (we assume Python 3.10 or higher).
+
+This software is dual licensed under the GNU General Public License
+(version 3 of the License, or (at your option) any later version,
+and also under a 3-clause BSD license. Recipients may choose which of these 
+licenses to use; please see the files gpl-3.0.txt and/or bsd_license.txt,
+respectively.
+
+This package can only be imported through a SCons script; as a result, it is
+generally not possible to inspect the docstrings interactively. Refer to the
+locally-built documentation or at https://pipelines.lsst.io/, or read the 
+source code instead.