"just a kindly reminder" is a perfectly correct and usable phrase in written English. You can use it whenever you need to remind someone of something in a polite or friendly way. For example: "Just a kindly reminder that the deadline for submitting your project is tomorrow."
And just a reminder, the headcount question reminded me.
So just a quick reminder.
"It was just a nice reminder.
"Just a little reminder to the guys," he said.
Just a friendly reminder about Palestine, at the Yonkers Metro North Station.
Just a little reminder, O.K. American golf is fine".
"It's just a harsh reminder that the worldwide economy continues to disappoint," Mr. Detrick said.
Just a friendly reminder, Futurama returns to Comedy Central tonight.
"Just a punctuality reminder.
15 Just a friendly reminder.
This is just a friendly reminder to look up.
When I feel like I can't trust my brain 100%, Ludwig really comes in handy. It makes me translate and proofread faster and my output more reliable.
Claudia Letizia
Head Translator and Proofreader @ organictranslations.eu