Add user horrortroll in userspace (#19769)

users/horrortroll/readme.md

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+/* Copyright 2023 HorrorTroll <https://github.com/HorrorTroll>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+# My personal keymap with custom LED
+
+### Custom LED effect list:
+
+- Custom gradient ([ported from SirTimmyTimbit code](https://github.com/SirTimmyTimbit/customizable-gradient-effect-for-drop-alt))
+- Cool diagonal ([ported from pleasuretek code](https://github.com/pleasuretek/qmk_firmware))
+- Flower Blooming
+- Random breath rainbow ([based from daed code](https://github.com/daed/qmk_firmware/blob/master/keyboards/massdrop/alt/keymaps/daed) and modify by me)