README: document optional dependencies a bit better

Now that `python3-mako` represents the first nontrivial dependency of a
main feature (e.g. `vulkan`), document this list properly.

Formatting could be bikeshed, but whatever. I just wanted to have a
list out there.

README.md

@@ -197,12 +197,18 @@ on distro packages.
 
 In principle, libplacebo has no mandatory dependencies - only optional ones.
 However, to get a useful version of libplacebo. you most likely want to build
-with support for either `opengl` or `vulkan`.
-
-libplacebo built without these can still be used (e.g. to generate GLSL
-shaders such as the ones used in VLC), but the usefulness is severely
-impacted since most components will be missing, impaired or otherwise not
-functional.
+with support for either `opengl` or `vulkan`. libplacebo built without these
+can still be used (e.g. to generate GLSL shaders such as the ones used in
+VLC), but the usefulness is severely impacted since most components will be
+missing, impaired or otherwise not functional.
+
+A full list of optional dependencies each feature requires:
+
+- **glslang**: `glslang` + its related libraries (e.g. `libSPIRV.so`)
+- **lcms**: `liblcms2`
+- **opengl**: `libepoxy`
+- **shaderc**: `libshaderc`
+- **vulkan**: `libvulkan`, `python3-mako`
 
 ### Configuring