use po4a to make all docs translatable on Weblate and others

This uses po4a to convert Markdown documents into gettext .po files, while
adding key metadata and hiding bits that the translators don't need to see.

For the deep dive, see:
https://guardianproject.info/2020/04/23/figuring-out-crowdsourced-translation-of-websites/

fsfe/reuse-website#26

.gitignore

@@ -0,0 +1,9 @@
+
+# no need to translate the source language, but po4a gens this file
+po/reuse-docs.en.po
+
+# po4a auto-generated markdown files from translations
+[a-z][a-z]/*.md
+[a-z][a-z][a-z]/*.md
+[a-z][a-z][a-z]_[A-Z]*/*.md
+[a-z][a-z]_[A-Z]*/*.md

README.md

@@ -34,6 +34,12 @@ We also accept and appreciate feedback by creating issues in the
 project or by sending an e-mail to the [public REUSE mailing
 list](https://lists.fsfe.org/mailman/listinfo/reuse).
 
+### Translation
+
+Translation happens by conversion Markdown into _gettext_ using
+[po4a](https://po4a.org).  To generate the _.md_ files from the
+_gettext .po_ files, run:  `po4a po/po4a.conf`.
+
 ## License
 
 The relevant documents in this repository are licensed under [Creative Commons Attribution-ShareAlike 4.0](https://creativecommons.org/licenses/by-sa/4.0).

po/po4a.conf

@@ -0,0 +1,12 @@
+[po4a_langs] es
+[po4a_paths] po/reuse-docs.pot $lang:po/reuse-docs.$lang.po
+
+[options] opt:"--addendum-charset=UTF-8" opt:"--localized-charset=UTF-8" opt:"--master-charset=UTF-8" opt:"--master-language=en_US" opt:"--msgmerge-opt='--no-wrap'" opt:"--porefs=file" opt:"--wrap-po=newlines"
+
+[po4a_alias:markdown] text opt:"--option markdown" opt:"--option yfm_keys=title" opt:"--addendum-charset=UTF-8" opt:"--localized-charset=UTF-8" opt:"--master-charset=UTF-8" opt:"--keep=100"
+
+[type: markdown] CHANGELOG.md $lang:$lang/CHANGELOG.md
+[type: markdown] README.md $lang:$lang/README.md
+[type: markdown] faq.md $lang:$lang/faq.md
+[type: markdown] spec.md $lang:$lang/spec.md
+[type: markdown] tutorial.md $lang:$lang/tutorial.md