Add some Generator documentation.

Author: oluwandabira

README.md

@@ -170,6 +170,22 @@ let config = {
 
 ```
 
+Possible Generator Triggers are:
+- `Once` : Runs once, globally.
+- `OnFile filename`: Run once for the given filename.
+- `OnFileExt extension` : Runs once for each file with the given extension.
+- `OnFilePredicate predicate` : Runs once for each file satisfying the predicate (`string -> string`).
+
+Possible Generator Outputs are:
+- `SameFileName` :  Output has the same filename as input file.
+- `ChangeExtension newExtension` : Output has the same filename but with extention change to `newExtension`.
+- `NewFileName newFileName` : Output filename is `newFileName`.
+- `Custom mapper` : Output filename is the result of applying the mapper to the input filename.
+- `MultipleFiles mapper` : Outputs multiple files, the names of which are a result of applying the mapper to the first string output of the generator.
+
+**Note**: For `MultipleFiles` the `generate` function *must* output a `list<string * string>`.
+
+
 ## How to contribute
 
 *Imposter syndrome disclaimer*: I want your help. No really, I do.

src/Fornax.Core/Model.fs

@@ -1327,17 +1327,23 @@ module Config =
         | Once
         ///Generator runs once, for given filename (file name is relative to project root, for example `post/post.md`). It runs only if the given file exist.
         | OnFile of filename : string
-        ///Generator runs for any file with given extension (for example `md`)
+        ///Generator runs for any file with given extension (for example `md`).
         | OnFileExt of extension: string
-        ///Generator runs for any file matching predicate. Parameters of predicate are absolut path to project root, and path to the file relative to project root.
+        ///Generator runs for any file matching predicate. Parameters of predicate are absolute path to project root, and path to the file relative to project root.
         | OnFilePredicate of predicate: ((string * string) -> bool)
 
     type GeneratorOutput =
+        ///Generates a file with the same name.
         | SameFileName
+        ///Generates a file with the same base name but with the extension changed.
         | ChangeExtension of newExtension: string
+        ///Generates a file with name `newFileName`.
         | NewFileName of newFileName: string
-        | Custom of (string -> string)
-        | MultipleFiles of (string -> string)
+        ///Generates a file with the name as the result of `mapper orignalFileName`.
+        | Custom of mapper: (string -> string)
+        ///Generates multiple files with each name being the result of applying the mapper to the first string of the generator output.
+        ///The generator must have a type `SiteContents -> string -> string -> list<string * string>`
+        | MultipleFiles of mapper: (string -> string)
 
     type GeneratorConfig = {
         Script: string