README.asciidoc: document use of "make check-htmlproofer*" recipes [n…

…etworkupstools#52]

Signed-off-by: Jim Klimov <[email protected]>

README.asciidoc

@@ -204,6 +204,24 @@ there. You probably need to commit that back to "source" github repository.
 NOTE: For hardcore maintainers, there should be a PGP/GPG key to also sign the
 release tarball, calling `make dist-sig-files` (would fail without a key).
 
+Sanity-checking the generated HTML files
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If the `htmlproofer` tool is installed (see above) and detected by the
+`configure` script (automatically for presence in `PATH`), you can explicitly
+call `make check-htmlproofer` to validate the files present in `OUTDIR_BASE`
+and/or `OUTDIR` (if stored separately from the base for custom or historic
+website builds).
+
+You can also validate the published website repository (into which you would
+upload the generated `OUTDIR` contents) as e.g. prepared by the `ci_build.sh`
+using `make check-htmlproofer-OUTDIR_PUBLISHED` (optionally customize the
+`OUTDIR_PUBLISHED` environment or `make` variable to point to the checkout
+location for that repository, if not using the scripted default).
+
+Note that this check can take about 10 minutes (especially if not disabling
+the referenced external site availability), so it is not done by default.
+
 Publishing
 ----------