Add issue_template.md file.

.github/issue_template.md

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+### Step 1: Provide a summary of your problem
+  * For general technical questions, problems or feature requests related to the code **in this repository** file an issue.
+
+### Step 2: Provide tmuxp details
+  * Python version
+  * system PATH
+  * tmux version
+  * tmuxp version
+  * tmux path
+  * tmuxp path,
+  * libtmux version
+  * shell
+  * output of `tmux list-sessions`, `tmux list-windows`, `tmux list-panes`
+  * output of `tmux show-options -g`, `tmux show-window-options -g`
+  * output of `tmuxp freeze <SESSION_NAME>`
+
+### Step 2: Describe your environment
+  * Architecture: _____
+  * OS version: _____
+
+### Step 3: Describe the problem:
+#### Steps to reproduce:
+  1. _____
+  2. _____
+  3. _____
+
+#### Observed Results:
+  * What happened?  This could be a description, log output, etc.
+
+#### Expected Results:
+  * What did you expect to happen?
+
+#### Relevant Code:
+  ```
+  // TODO(you): paste here tmuxp configuration (YAML / JSON) you are having issues with.