add some porting notes

docs/development.md

@@ -54,6 +54,7 @@ Platform-specific instructions:
 - [Windows](./windows.md)
 - [Mac](./mac.md)
 - [Linux](./linux.md)
+- [Other Platforms](./new-platform.md)
 
 Don't name the Git checkout ~/Anki or ~/Documents/Anki, as those folders
 were used on old Anki versions and will be automatically moved.

docs/new-platform.md

@@ -0,0 +1,63 @@
+# Building on a new platform
+
+## Upstream prerequisites
+
+- A recent Qt, PyQt5 and PyQtWebEngine must work on your platform. Chances are wheels
+  for your platform are not available on PyPI, so you will need to have a working
+  version installed already, either from your Linux distro's archives, or by building
+  from source.
+- Rust and Node must support your platform.
+
+## 32 bit builds
+
+Bazel does not support running on a 32-bit host. It may be theoretically
+possible to cross-compile from a 64-bit host, but this is not something we've
+tried, and it will likely not work out of the box.
+
+## Protobuf & Python
+
+- Edit /platforms/BUILD.bzl and add the new platform. Available platforms
+  and CPUs can be found here: https://github.com/bazelbuild/platforms
+- Edit /protobuf.bzl and add the new platform and a link to a protoc binary.
+- Edit /pylib/anki/BUILD.bazel and add the new platform and the relevant
+  Python wheel name for your platform.
+
+Example of the above changes:
+https://github.com/ankitects/anki/commit/db3308e788f20b188e84add40d6a1dce5bf726a0
+
+- We need to tell the build process to use your local PyQt5 install, instead of
+  fetching it from PyPI. Locate your Python site-packages folder where the PyQt5
+  folder is, and add the following to user.bazelrc in the project folder root:
+
+```
+build --action_env=PYTHON_SITE_PACKAGES=/path/to/site-packages
+```
+
+- Anki uses the Python 'orjson' module. If it's not available on your system,
+  you will need to [patch it out](https://github.com/ankitects/anki/pull/752#issuecomment-748861582), and remove references to it in the build scripts.
+
+## Rust
+
+- Ensure you have Rust installed (eg via rustup)
+- Edit /Cargo.toml in this project, and add your platform
+  to the targets. Check that the platform [is supported](https://github.com/bazelbuild/rules_rust/blob/master/rust/platform/platform.bzl) by rules_rust; if it is not, you'll
+  need to send a PR to them adding it. You can test your change locally by modifying
+  /repos.bzl in this project to point to a local rules_rust checkout.
+- Edit /cargo/BUILD.request.bazel and add the new platform.
+- Run update.py in the /cargo folder.
+
+Examples of the required changes:
+
+- https://github.com/ankitects/anki/commit/eca27b371000e77b68cb4c790b44848507ca3883
+- https://github.com/ankitects/anki/commit/3f3f4b5c3640a7d1f4eec02f326fda93214ec34b
+
+## NodeJS
+
+If you node don't provide a binary for your platform and you have a local copy
+installed, you may be able to modify node_repositories() in /defs.bzl to point
+to your [local npm installation](https://bazelbuild.github.io/rules_nodejs/install.html).
+
+## Submitting changes
+
+If the changes to support your platform do not require platform-specific hacks,
+a PR that adds them is welcome - please see [Contributing](./contributing.md) for more.