Compiler: add some doc on plugin support

CONTRIBUTING.md

@@ -158,6 +158,13 @@ To add support for a new language:
 Feel free to open a pull request for discussion even if you couldn't go through
 all these steps, the `lexer_xx.cppo.ml` file is the important part.
 
+### Example: writing custom backends as plugins
+
+Catala has support for dynamically-loaded plugins to use as alternative
+backends. See `compiler/plugins` for examples, and [the
+documentation](https://catala-lang.org/ocaml_docs/catala/plugins.html) for more
+detail.
+
 ### Automatic formatting
 
 Please ensure to submit commits formatted using the included `ocamlformat`

compiler/index.mld

@@ -99,9 +99,20 @@ verification condition for proof backends. More information can be found here:
 {li {{: verification.html} Verification}}
 }
 
-Last, two more modules contain additional features for the compiler:
+Two more modules contain additional features for the compiler:
 
 {ul
 {li {{: literate.html} Literate programming}}
 {li {{: utils.html} Compiler utilities}}
 }
+
+Last, it is possible to customize the backend to the compiler using a plugin
+mechanism. The API is defined inside the following module:
+
+{!modules: Driver.Plugin}
+
+See the examples in the [plugins/] subdirectory:
+
+{ul
+{li {{: plugins.html} Backend plugin examples}}
+}

compiler/plugins/dune

@@ -9,3 +9,7 @@
  (modes plugin)
  (modules jsoo)
  (libraries catala.driver catala.runtime))
+
+(documentation
+ (package catala)
+ (mld_files plugins))

compiler/plugins/plugins.mld

@@ -0,0 +1,51 @@
+{0 Example backend plugins }
+
+This directory contains backend plugins that demonstrate how those can be
+written and used with Catala. They probably don't provide much value otherwise.
+
+Use [make plugins] from the root of the source tree to build them.
+
+A plugin is created by writing an OCaml module that calls
+[Driver.Plugin.register_lcalc] or [Driver.Plugin.register_scalc] and that links
+against [catala.driver]. Here is an example dune stanza to compile it:
+{v
+(executable
+ (name my-plugin)
+ (modes plugin)
+ (modules my_plugin_main_module)
+ (libraries catala.driver))
+v}
+
+See the following module for the registration interface:
+
+{!modules: Driver.Plugin}
+
+{1 Using plugins}
+
+Plugins are dynamically loaded. The Catala compiler will be looking for them at
+startup within [<prefix>/lib/catala/plugins] (assuming that the compiler is
+installed into [<prefix>/bin]), or any other directory specified through the
+`--plugin-dir` command-line flag or by setting the [CATALA_PLUGINS] environment
+variable.
+
+The plugin of your choice can then be called just like the built-in backends, using:
+{v
+$ catala MyPlugin <file> [options]
+v}
+
+{1 Examples}
+
+{2 python example}
+
+This trivial example registers a plugin that uses the [scalc] format as input.
+It simply calls the code of the built-in Python backend, and should be no
+different to using the normal Catala Python output mode.
+
+{2 jsoo example}
+
+This slightly more involved plugin reads the [lcalc] format, applies the code of
+the [OCaml] backend normally, but then calls the [ocamlc] and [js_of_ocaml]
+compiler successively on the output in order to give a Javascript output.
+
+Note that this output remains a library, it won't provide user-facing features,
+and no efforts are made to make it callable from normal JavaScript code.