[gnomemm-website] Update README and rename to README.md

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 4150f43cf163c4e11344935cbd763047ef8da360
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Thu Jan 27 14:43:11 2022 +0100

    Update README and rename to README.md

 README    | 39 ---------------------------
 README.md | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 91 insertions(+), 39 deletions(-)
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+# Gtkmm's website
+
+This is the source of the [gtkmm website](https://www.gtkmm.org).
+
+The source consists of a DocBook 5.0 document.
+It requires the docbook5-xml and docbook-xsl-ns packages (Ubuntu names,
+can have other names in other distros). It may be possible to build without
+these packages, but it will be __much__ slower (minutes instead of seconds).[^1]
+
+The website can be built with Meson or Autotools.
+
+## Building with Meson
+
+- Create a build directory and configure it:
+  ```sh
+  cd gnomemm-website
+  meson [options] <build-dir>
+  ```
+  Do not call the build-dir gnomemm-website/build. There is already such a
+  directory, used when building with Autotools.
+
+- Create the html files of the website:
+  ```sh
+  cd <build-dir>
+  ninja
+  ```
+
+- Watch the result:
+  - Open \<build-dir>/docs/html/index.html in a web browser.
+
+- Publish the website at www.gtkmm.org (requires a login at www.gtkmm.org):
+  ```sh
+  ninja publish
+  ```
+  At the time of writing (2022-01-27) the `publish` target is disabled.
+  You only get instructions how to enable it.
+
+# Building with Autotools
+
+- Configure, build in the source directory:
+  ```sh
+  cd gnomemm-website
+  ./autogen.sh
+  ```
+
+- Create the html files of the website:
+  ```sh
+  make
+  ```
+
+- Watch the result:
+  - Open docs/html/index.html in a web browser.
+
+- Publish the website at www.gtkmm.org (requires a login at www.gtkmm.org):
+  ```sh
+  cd docs
+  make post-html
+  ```
+
+## Adding a new language
+
+- Generate a temporary .pot file and the .po file for your locale, say xx_XX:
+  ```sh
+  cd docs
+  itstool -o gnomemm-website.pot C/index.docbook C/*.xml
+  mkdir xx_XX
+  msginit -i gnomemm-website.pot -o xx_XX/xx_XX.po -l xx_XX.utf8
+  ```
+
+- Translate the .po file.
+
+- Modify docs/C/index.docbook:
+  - Find `<formalpara xml:id="language-menu">`, and add your locale.
+    ```html
+    <listitem>
+      <para><link xlink:href="../xx_XX/index.html">Xxxx</link></para>
+    </listitem>
+    ```
+
+- Add your language to docs/LINGUAS (used when building with Meson).
+
+- Add your language to HELP_LINGUAS in docs/Makefile.am (used when building with Autotools).
+
+- Run `make` or `ninja` to generate all pages.
+
+--------------------
+<!-- footnote -->
+[^1]: The `xmllint` command is told to read files from `http://docbook.org`.
+The `xsltproc` command is told to read files from `http://docbook.sourceforge.net`.
+The commands first search for local copies of those files. If local copies exist
+and are installed at expected locations, the commands make no network accesses.