Add ISSUE_TEMPLATE.md

Close #500

.github/ISSUE_TEMPLATE.md

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+<!-- Check all that apply [x] -->
+- Category
+    - [ ] fzf binary
+    - [ ] Key bindings
+    - [ ] Completion
+    - [ ] Vim
+    - [ ] Neovim
+    - [ ] Etc.
+- OS
+    - [ ] Linux
+    - [ ] Mac OS X
+    - [ ] Windows
+    - [ ] Etc.
+- Shell
+    - [ ] bash
+    - [ ] zsh
+    - [ ] fish
+
+<!--
+### Before submitting
+
+- Make sure that you have the latest version of fzf
+- If you use tmux, make sure $TERM is set to screen or screen-256color
+- For more Vim stuff, check out https://github.com/junegunn/fzf.vim
+
+Describe your problem or suggestion from here ...
+-->
+