Mention language pack for Gnome in i18n.md

[antonsoroko]

$ ./tools/po-update.sh ru
/usr/bin/msgunfmt: error while opening "/usr/share/locale-langpack/ru/LC_MESSAGES/gtk30.mo" for reading: No such file or directory
/usr/bin/msgunfmt: error while opening "/usr/share/locale-langpack/ru/LC_MESSAGES/gtk30-properties.mo" for reading: No such file ordirectory
/usr/bin/msgmerge: error while opening "" for reading: No such file or directory

$ ./tools/po-update.sh en
Created /home/antix/work/Solaar/po/en.po.
/usr/bin/msgunfmt: error while opening "/usr/share/locale-langpack/en/LC_MESSAGES/gtk30.mo" for reading: No such file or directory
/usr/bin/msgunfmt: error while opening "/usr/share/locale-langpack/en/LC_MESSAGES/gtk30-properties.mo" for reading: No such file ordirectory
/usr/bin/msgmerge: error while opening "" for reading: No such file or directory

$ apt-file search /usr/share/locale-langpack/ru/LC_MESSAGES/gtk30.mo
language-pack-gnome-ru-base: /usr/share/locale-langpack/ru/LC_MESSAGES/gtk30.mo

$ apt-file search /usr/share/locale-langpack/en/LC_MESSAGES/gtk30.mo
language-pack-gnome-en-base: /usr/share/locale-langpack/en/LC_MESSAGES/gtk30.mo

thus documentation must be updated.

[pfps]

Seems reasonable.