software/ors-amarisoft: slapos-render-config: Document what this program does

When I was first starting to look around inside ors-amarisoft software
release for me it was not clear off-hand what this program is for. It
should be more clear about what's the intent if there is explicit
comment.

/cc @xavier_thompson, @Daetalus
/reviewed-by @jhuge, @lu.xu, @tomo
/reviewed-on nexedi/slapos!1462

software/ors-amarisoft/slapos-render-config.py

+# Program slapos-render-config is handy during config/ templates development.
+#
+# It mimics the way config files are generated during the build but runs much
+# faster compared to full `slapos node software` + `slapos node instance` runs.
+
 config = "gnb"
 json_params_empty = """{
     "rf_mode": 'fdd',