This web app lets you write quick notes in japanese, allowing you to add furigana on any kanji you need.
You can try it at https://daphyre.github.io/furigana-notes
To create furigana over words you can add square brackets around the characters you want to mark, and then parenthesis with the furigana for this word. For example:
[日本語](にほんご)
Will become:
日本語
Some older browsers might not support furigana, in which case, the text will just be displayed between parenthesis.
If your keyboard supports full width parenthesis, you can use them instead to put furigana to the character before it. For example:
日（に）本（ほん）語（ご）
Will become:
日本語