Add basic building instructions (#77)

* Basic instructions for building

* One more flag and info for keeping up to date

* Move BUILDING.md to doc/

* Initial section on packaging. Small changes.

* Fix link from Readme

Co-authored-by: Simon-L <lumis@xulepth.fr>

README.md

@@ -63,6 +63,8 @@ Currently the following features are known NOT to work:
 If you want to try this out early, checkout the [GitHub actions tab](https://github.com/DISTRHO/Cardinal/actions/workflows/build.yml).  
 There is absolutely no warranty, use at your own risk and all that...
 
+Basic building instructions are available in [BUILDING.md](doc/BUILDING.md)
+
 ### Community chat
 
 Currently we are all on #cardinal IRC room in irc.libera.chat server.  

doc/BUILDING.md

@@ -0,0 +1,49 @@
+# Building
+
+> Note that you can likely also get a build directly by logging in to Github and heading to our [CI builds page](https://github.com/DISTRHO/Cardinal/actions/workflows/build.yml), the latest successful build has the files at the bottom of the page.
+
+To build Cardinal locally on Debian-based Linux, you need a few dependencies (taken from the Github CI [build.yml](.github/workflows/build.yml)):
+
+`sudo apt install libgl1-mesa-dev liblo-dev libx11-dev libxcursor-dev libxext-dev libxrandr-dev`
+
+You also need a basic toolchain and `cmake`:
+
+`sudo apt install cmake build-essential`
+
+It's important to clone the repo including all submodules:
+
+`git clone https://github.com/DISTRHO/Cardinal --recursive`
+
+You can simply build using:
+
+`make -j 2`
+
+To quickly test you build setup you can run `make DEBUG=true NOPLUGINS=true -j $(nproc)`
+
+The build will be in `bin/` where you should have vst2, vst3, lv2 and Jack standalone of all 3 compatible [variants](../README.md#plugin-variants) Full, FX and Synth.  
+The plugins expect to remain *within* their parent folder. If you move them around make sure to keep this structure.  
+You can alternatively use symlinks from the user-specific locations (this is recommended), e.g. for lv2:
+`ln -s <path to repo>/bin/Cardinal.lv2 ~/.lv2/Cardinal.lv2`
+
+There are a few build flags to know about, use them as `FLAG=true` or `false`:
+
+* `DEBUG=true` to enable debugging
+* `NOPLUGINS=true` to only build the Cardinal Core plugins for audio and Midi I/O and the fancy blank panel
+* `WITH_LTO=true` to enable Link Time Optimization, this significantly increases the build time
+* `SKIP_STRIPPING=true` to disable stripping the binaries if you don't need a full debug
+
+#### Keeping up to date
+
+Things are evolving quickly in Cardinal! To keep your local copy up to date with the changes, do:  
+```
+git pull
+git submodule update --init --recursive
+```
+
+### Packaging/distributing
+
+If you are a package maintainer, use the flag `SYSDEPS=true` to use the system dependencies.  
+
+Currently, the only existing packages we are aware of are in Arch's AUR: [cardinal.lv2-git](https://aur.archlinux.org/packages/cardinal.lv2-git/) and [cardinal-git](https://aur.archlinux.org/packages/cardinal-git/).
+
+Feel free to get in touch via Discussions tab or on [IRC](../README.md#community-chat)