Add a list of recommended plugins (all free/libre)

content/appendices/plugins/index.en.md

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+title = "Recommended plugins"
+description = "Plugins we recommend to use with Ardour"
+chapter = false
+weight = 6
++++
+
+## Filters and processing
+
+- [LSP plugins](https://lsp-plug.in/), an extensive set of plugins for effects and channel processing
+- [dpl.lv2](https://x42-plugins.com/x42/x42-limiter), a look-ahead digital peak limiter
+- [fil4.lv2](https://x42-plugins.com/x42/x42-eq), a 4 band parametric equalizer with additional low & high shelfing and high & low-pass filters
+
+<!-- - [airwindows](https://www.airwindows.com/) -->
+
+## Visualization
+
+- [x42 Meter Collection](https://x42-plugins.com/x42/x42-meters), a big help in analyzing audio
+- [sisco.lv2](https://x42-plugins.com/x42/x42-scope), an oscilloscope
+- [spectra.lv2](https://x42-plugins.com/x42/x42-spectra), a spectrum analyzer
+
+## Drums and percussion
+
+- [AVL Drumkits](https://x42-plugins.com/x42/x42-avldrums), a drum sampler with two built-in kits
+- [DrumGizmo](https://drumgizmo.org), a drum sampler with separately downloadable kits
+- [Geonkick](https://github.com/free-sm/geonkick), a drums/percussion synthesizer
+
+## Synthesizers
+
+- [Cardinal](https://github.com/DISTRHO/Cardinal), a modular synthesizer with Eurorack aesthetics
+- [OB-Xd](https://github.com/reales/OB-Xd), a soft synth resembling vintage Oberheim OB-X synth
+- [Dexed](https://github.com/asb2m10/dexed), a Yamaha DX7 simulator
+- [Odin2](https://www.thewavewarden.com/odin2), a 24-voice polyphonic synth
+- [Surge XT](https://surge-synthesizer.github.io/), a hybrid synthesizer
+- [Vital](https://vital.audio/), a spectral warping wavetable synth
+<!-- - [Helm](https://tytel.org/helm/) -->
+
+[This website](http://linuxsynths.com/) keeps a track of all software
+synthesizers available for Linux.