Add OpenTechCon (#1152)

* Add OpenTechCon 2024/09/28

* Update README.md

---------

Co-authored-by: Aurelie Vache <scraly@gmail.com>

README.md

@@ -185,6 +185,7 @@ All the data (past and coming) are available publicly in JSON:
 * 28: [RomHack](https://romhack.io/) - Rome (Italy) <a href="https://romhack.io/call-for-papers/"><img alt="CFP RomHack 2024" src="https://img.shields.io/static/v1?label=CFP&message=until%2031-May-2024&color=red"></a>
 * 28: [ThreatModCon 2024 San Francisco](https://www.threatmodcon.com/) - San Francisco, CA (USA) <a href="https://sessionize.com/threatmodcon-2024-san-francisco"><img alt="CFP ThreatModCon 2024 San Francisco" src="https://img.shields.io/static/v1?label=CFP&message=until%2015-May-2024&color=red"></a>
 * 28: [SQL Saturday Minnesota](https://sqlsaturday.com/2024-09-28-sqlsaturday1087/) - Saint Paul, MN (USA)
+* 28: [OpenTechCon](https://opentechcon.fr/) - Online
 * 29-03/10: [MIC Atlanta](https://militaryinfluencer.com/) - Atlanta, GA (USA) <a href="https://sessionize.com/mic-atlanta/"><img alt="CFP MIC Atlanta" src="https://img.shields.io/static/v1?label=CFP&message=until%2020-April-2024&color=red"></a>
 * 30-01/10: [Targeting Quality 2024](https://targetingquality.ca/) - Cambridge, ON (Canada) <a href="https://www.papercall.io/tq2024"><img alt="CFP Targeting Quality 2024" src="https://img.shields.io/static/v1?label=CFP&message=until%2017-April-2024&color=red"></a>
 * 30-01/10: [Open Source in Finance Forum](https://events.linuxfoundation.org/open-source-finance-forum-new-york/) - New York, NY (USA) <a href="https://events.linuxfoundation.org/open-source-finance-forum-new-york/program/cfp/"><img alt="CFP OSFF" src="https://img.shields.io/static/v1?label=CFP&message=until%2014-July-2024&color=red"></a>