Add a proper roadmap

Makes sense to have this as part of the repo.

ROADMAP.md

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+# Roadmap
+
+Yabridge's VST2 and VST3 bridging are feature complete and should work great,
+but there are still some other features that may be worth implementing. This
+page lists some of those.
+
+# Short-ish term
+
+- [ARA](https://www.celemony.com/en/service1/about-celemony/technologies)
+  support for VST3 plugins. The ARA SDK has recently been [open
+  source](https://github.com/Celemony/ARA_SDK), so we can now finally start
+  working on this.
+
+# Longer term
+
+- [CLAP](https://github.com/free-audio/clap) plugin bridging. Implementing this
+  only makes sense once Windows-only CLAP plugins start appearing.
+- An easier [updater](https://github.com/robbert-vdh/yabridge/issues/51) through
+  a new `yabridgectl update` command for distros that don't package yabridge.
+
+# For a major release
+
+- Completely remove the now deprecated symlink-based installation method from
+  yabridgectl.
+- Replace the use of `notify-send` for notifications with using `libdbus`
+  directly. Most systems will have both available by default, but some less
+  common distros split `notify-send` from the rest of the `libnotify` package.
+- Possibly combine the `yabridge-host` and `yabridge-group` binaries to save
+  some disk space as 95% of their code overlaps.
+- Consider adding an option for yabridgectl to set up VST2 plugins in `~/.vst`.
+  As discussed in a couple places already doing so would come with a number of
+  downsides and potential pitfalls so this may not happen.
+- Consider chainloading the real `libyabridge-vst2.so` and `libyabridge-vst3.so`
+  files from smaller stub libraries. This would avoid having to rerun
+  `yabridgectl sync` after an upgrade, and it would save disk space on systems
+  without support for reflinks.
+
+# Somewhere in the future, possibly
+
+- REAPER's vendor specific [VST2.4](https://www.reaper.fm/sdk/vst/vst_ext.php)
+  and
+  [VST3](https://github.com/justinfrankel/reaper-sdk/blob/main/sdk/reaper_vst3_interfaces.h)
+  extensions.
+- [Presonus' extensions](https://presonussoftware.com/en_US/developer) to the
+  VST3 interfaces. All of these extensions have been superseded by official VST3
+  interfaces in later versions of the VST3 SDK, so it's unlikely that there are
+  many plugins that still rely on these older extensions.