Add plantuml target and explain how to build docs

Makefile

@@ -13,7 +13,14 @@ help:
 
 .PHONY: help Makefile
 
+plantuml:
+	# -o is relative to the source dir.
+	plantuml -v -tsvg -o _static/ source/*.puml
+
+.PHONY: plantuml
+
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	# -make plantuml
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)

README.rst

@@ -3,3 +3,10 @@ pEp MixMailer documentation
 
 Work in progress documentation about pEp "MixMailer" project including
 analysis, design and specification.
+
+To build the documentation, install `Sphinx <https://www.sphinx-doc.org>`_ and
+run ``make html``.
+Optionally, you can build new .svg diagrams from .puml files installing
+`plantUML <https://plantuml.com>` and running make plantuml.
+
+Authors: juga at riseup dot net.