Not too smooth localization process

[yfdyh000]

This is some suggestions, I failed to do some because I unskilled the code and community.

1. The Currently Shaarli has no translation/internationalization/localization system available and is single-language. are outdated in CONTRIBUTING.md. Update it and point to doc/md/dev/Translations.md (or https://shaarli.readthedocs.io/en/master/dev/Translations/).
2. Did not mention the application\Languages.php for adding new locale in Translations.md.
3. "Open inc/languages/<lang>/LC_MESSAGES/shaarli.po under Shaarli's directory" is confusing when "Add a new language", I finally tried anyone language ("de"), "save as" and updated from the source code, which was lucky.
4. I have encountered an unexpected errors for "Plural-Forms", the C:\Program Files (x86)\Poedit\GettextTools\bin\msgcat.exe: msgid 'Search' is used without plural and with plural., it blocks updating po(t) file from code. I finally resolve it by excluding files (see the X-Poedit-SearchPathExcluded of Add Chinese (Simplified) translation #1781). Other locales (like 'ja') may have similar case.
5. This script doesn't seem to work for me, put and access it, I didn't get a good result, I eventually access the pages listed in that document one by one. I recommend integrating this script to the /shaarli/inc directory.
6. Provides a more one-click deployment script / steps for various OS / cloud platform, it may be better.

[ArthurHoaro]

Thank you for your feedbacks. I mostly wrote this documentation when I wrote the translation system (and French translation) a while ago, so it's not the most user friendly. We should definitely address your points.