[#29] Add criteria for language inclusion

README.md

@@ -13,6 +13,77 @@ etc.)**
 This project is a work in progress, and I don't intend on thoroughly
 documenting it until it has reached feature-completeness.
 
+## Criteria for language inclusion
+
+I aspire for Riju to support more languages than any reasonable person
+could conceivably think is reasonable. That said, there are some
+requirements:
+
+* **Language must have a clear notion of execution.** This is because
+  a core part of Riju is the ability to execute code. Languages like
+  [YAML](https://yaml.org/), [SCSS](https://sass-lang.com/), and
+  Markdown are fine because they have a canonical transformation (into
+  [JSON](https://www.json.org/json-en.html),
+  [CSS](https://developer.mozilla.org/en-US/docs/Web/CSS), and
+  [HTML](https://developer.mozilla.org/en-US/docs/Web/HTML)
+  respectively) that can be performed on execution. However, languages
+  like JSON, CSS, and HTML are not acceptable, because there's nothing
+  reasonable to do when they are run.
+* **Language must not require input or configuration.** This is
+  because, in order to avoid bloating the interface, Riju provides a
+  way to supply code but not any other data. Of course, it's possible
+  to supply input interactively, so reading stdin is allowed, but if a
+  language can only reasonably be programmed with additional input,
+  it's not a candidate for inclusion. Thus, many templating languages
+  are excluded, since they don't do anything unless you are
+  substituting a value. However, some languages such as
+  [Pug](https://pugjs.org/) are allowed, because they implement a
+  significant syntax transformation outside of template substitution.
+  Also, languages like [Sed](https://www.gnu.org/software/sed/) and
+  [Awk](https://www.gnu.org/software/gawk/) are allowed, because it's
+  straightforward to test code written in them even without a
+  pre-prepared input file.
+* **Language must not require a graphical environment.** This is
+  because we use a pty to run code, and there is no X forwarding. As
+  such, we can't use languages like
+  [Scratch](https://scratch.mit.edu/),
+  [Alice](https://www.alice.org/), and
+  [Linotte](http://langagelinotte.free.fr/wordpress/).
+* **Language must be available for free under a permissive license.**
+  This is because we must download and install all languages
+  noninteractively in the Docker image build, so anything that
+  requires license registration is unlikely to work (or be legal). We
+  can't use [Mathematica](https://www.wolfram.com/mathematica/) or
+  [MATLAB](https://www.mathworks.com/products/matlab.html), for
+  example, but we can use [Mathics](https://mathics.github.io/) and
+  [Octave](https://www.gnu.org/software/octave/), which provide
+  compatible open-source implementations of the underlying languages.
+* **Language must be runnable under Docker on Linux.** This is because
+  that's the execution environment we have access to.
+  [AppleScript](https://en.wikipedia.org/wiki/AppleScript) is out
+  because it only runs on macOS, and [Docker](https://www.docker.com/)
+  is out because it can't be run inside Docker (without the
+  `--privileged` flag, which has unacceptable security drawbacks; see
+  [#29](https://github.com/raxod502/riju/issues/29)). Note, however,
+  that many Windows-based languages can be used successfully via
+  [Mono](https://www.mono-project.com/) or
+  [Wine](https://www.winehq.org/), such as
+  [Cmd](https://en.wikipedia.org/wiki/Cmd.exe),
+  [C#](https://en.wikipedia.org/wiki/C_Sharp_(programming_language)),
+  and [Visual Basic](https://en.wikipedia.org/wiki/Visual_Basic).
+
+Here are some explicit *non-requirements*:
+
+* *Language must be well-known.* Nope, I'll be happy to add your pet
+  project; after all, [Kalyn](https://github.com/raxod502/kalyn) and
+  [Ink](https://github.com/thesephist/ink) are already supported.
+* *Language must be useful.* I would have no objection to adding
+  everything on the esolangs wiki, if there are interpreters/compilers
+  available.
+* *Language must be easy to install and run.* Well, it would be nice,
+  but I've seen some s\*\*\* when adding languages to Riju so it will
+  take a lot to surprise me at this point.
+
 ## Project setup
 
 To run the webserver, all you need is Yarn. Just run `yarn install` as