Update issue templates

.github/ISSUE_TEMPLATE/headphone-requests.md

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+---
+name: Headphone Requests
+about: Your headphone missing from the list?
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**Headphone requests**
+Headphone requests are not welcome. If the headphones have not been measured by one of the supported sources, AutoEq cannot support it. If you happen to find a frequency response graph for the headphones, you can create eq settings yourself by following [this guide](https://medium.com/@jaakkopasanen/make-your-headphones-sound-supreme-1cbd567832a9). Open a headphone request issue only if the headphones have been measured by one of the supported sources but have not been added to AutoEq.