Create restoring_bash_history.md

docs/restoring_bash_history.md

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+tmux-ressurect no longer restores shell history for each pane, as of [this PR](https://github.com/tmux-plugins/tmux-resurrect/pull/308).
+
+As a workaround, you can use the `HISTFILE` environment variable to preserve history for each pane separately, and modify
+`PROMPT_COMMAND` to make sure history gets saved with each new command.
+
+Unfortunately, we haven't found a perfect way of getting a unique identifier for each pane, as the `TMUX_PANE` variable
+seems to occasionally change when resurrecting. As a workaround, the example below sets a unique ID in each pane's `title`.
+The downside of this implementation is that pane titles must all be unique across sessions/windows, and also must use the `pane_id_prefix`.
+
+Any improvements/suggestions for getting a unique, persistent ID for each pane are welcome!
+
+```bash
+pane_id_prefix="resurrect_"
+
+# Create history directory if it doesn't exist
+HISTS_DIR=$HOME/.bash_history.d
+mkdir -p "${HISTS_DIR}"
+
+if [ -n "${TMUX_PANE}" ]; then
+
+  # Check if we've already set this pane title
+  pane_id=$(tmux display-message -p '#{pane_title}')
+  if [[ $pane_id != "$pane_id_prefix"* ]]; then
+
+    # if not, set it to a random ID
+    random_id=$(head /dev/urandom | tr -dc A-Za-z0-9 | head -c 16)
+    printf "\033]2;$pane_id_prefix$random_id\033\\"
+    pane_id=$(tmux display-message -p '#{pane_title}')
+  fi
+
+  # use the pane's random ID for the HISTFILE
+  export HISTFILE="${HISTS_DIR}/bash_history_tmux_${pane_id}"
+else
+  export HISTFILE="${HISTS_DIR}/bash_history_no_tmux"
+fi
+
+# Stash the new history each time a command runs.
+export PROMPT_COMMAND="$PROMPT_COMMAND;history -a"
+```