docs: add some notes on using Vagrant to create troubleshooting envs

CONTRIBUTING.md

@@ -3,3 +3,73 @@
 Patches are welcome via the usual mechanisms (pull requests, email, posting to the project issue tracker etc).
 
 For more details, see the "command-t-development" section in [the documentation](https://github.com/wincent/command-t/blob/main/doc/command-t.txt).
+
+# Reproducing bugs
+
+Sometimes [user bug reports](https://github.com/wincent/command-t/issues) depend on characteristics of their local setup. Reproducing this may require copying configuration and installing dependencies, something I'd rather not do to my own development system. So, here are some notes about setting up Vagrant on macOS to provide a disposable VM on which to try things out in a controlled environment.
+
+## Installing Vagrant and VirtualBox
+
+```bash
+brew install vagrant
+brew install --cask virtualbox
+```
+
+## Creating a Vagrant VM
+
+```bash
+vagrant init hashicorp/bionic64
+vagrant up
+vagrant ssh
+```
+
+### Trouble-shooting Vagrant issues
+
+There are lots of things that can go wrong, so here are a few links:
+
+- ["There was an error while executing `VBoxManage`"](https://stackoverflow.com/a/51356705/2103996).
+- ["Vagrant up error while executing ‘VBoxManage’"](https://discuss.hashicorp.com/t/vagrant-up-error-while-executing-vboxmanage/16825).
+
+Which, among other things suggest these possible fixes:
+
+```bash
+sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart
+vagrant destroy -f
+vagrant box remove hashicorp/bionic64
+rm ~/Library/VirtualBox
+```
+
+For me, removing `~/Library/VirtualBox` did the trick.
+
+## Setting up Neovim on the VM
+
+```bash
+sudo apt-get update
+sudo apt-get install -y neovim # It's v0.2.2 🤦 — not going to be much help, so...
+
+sudo apt-get install -y cmake gettext libtool libtool-bin pkg-config unzip # instead...
+git clone https://github.com/neovim/neovim
+cd neovim
+make CMAKE_BUILD_TYPE=RelWithDebInfo
+sudo make install
+```
+
+## Installing Command-T and other dependencies
+
+```bash
+mkdir -p ~/.config/nvim/pack/bundle/start
+git clone https://github.com/wincent/command-t ~/.config/nvim/pack/bundle/start/command-t
+echo "require('wincent.commandt').setup()" > ~/.config/nvim/init.lua
+(cd /home/vagrant/.config/nvim/pack/bundle/start/command-t/lua/wincent/commandt/lib && make)
+
+# Also install any other plug-ins that might be needed to reproduce a problem; eg:
+git clone https://github.com/jiangmiao/auto-pairs ~/.config/nvim/pack/bundle/start/auto-pairs
+```
+
+## Cleaning up after testing is done
+
+```bash
+exit
+vagrant halt
+vagrant destroy
+```