docs: update build.md for spack

Adding some basic docs to indicate how to build ParaView from the
current source checkout using spack.

Documentation/dev/build.md

@@ -473,6 +473,47 @@ The following targets are used to build documentation for ParaView:
   * `ParaViewPythonDoc` - build the documentation from ParaView's Python source files.
   * `ParaViewDoc-TGZ` - build a gzipped tarball of ParaView documentation.
 
+## Using spack
+
+[Spack][spack] is a package manager for supercomputers, Linux and macOS. ParaView is one of
+the packages available in Spack. To install ParaView from spack, you can use:
+
+```
+spack install paraview
+```
+
+Please refer to [Spack documentation][spack-docs] for ways of customizing the install,
+including choosing the version and/or variant to build. Based one the version chosen,
+spack will download appropriate ParaView source and build it.
+
+To make it easier to build ParaView using spack from an existing source checkout, we have included
+relevant spack `package.yaml` files within the ParaView codebase itself. This
+also makes it easier to keep the spack package up-to-date with any changes to
+the ParaView buildsystem. With every release (and as frequently as required), we
+will push the changes to the ParaView paraview.yaml file upstream to the
+official spack repository.
+
+To build your existing source checkout of ParaView using Spack, here are the
+steps:
+
+```bash
+# assuming you've installed spack as documented in spack docs
+# and activate the spack environment appropriately
+
+# add custom paraview/package.yaml
+> spack repo add $PARAVIEW_SOURCE_DIR/Utilities/spack/repo
+
+# use info to confirm that the paraview package is available
+# only one version should be available
+> spack info paraview
+
+# install similar to any other spack package
+# e.g. following command installs osmesa-capable ParaView
+# with mpich
+
+> spack install paraview+osmesa^mesa~glx^mpich
+```
+
 [cmake-download]: https://cmake.org/download
 [cmake]: https://cmake.org
 [ffmpeg]: https://ffmpeg.org
@@ -493,3 +534,5 @@ The following targets are used to build documentation for ParaView:
 [qt-download-5.12.3]: https://download.qt.io/archive/qt/5.12/5.12.3/
 [tbb]: https://github.com/intel/tbb/releases
 [visual-studio]: https://visualstudio.microsoft.com/vs/older-downloads
+[spack]: https://spack.io/
+[spack-docs]: https://spack.readthedocs.io/en/latest/