chore: added readme.session.md for my future self

Author: Bilb

README.session.md

@@ -0,0 +1,65 @@
+# Sharp-libvips in Session
+
+This is our own fork of `sharp-libvips`, with some changes to make it work with our specific needs.
+Here are some details of how sharp and `sharp-libvips` are architected, so if someone has to come back to this project in 2 years, they can understand what's going on.
+
+## High level overview
+
+`sharp-libvips` is a project to clone and build all of the libvips dependencies.
+Then sharp will use the prebuilt binaries to build a sharp.node module.
+
+It generates through the CI a libvips for each supported platform and architecture. This step is very slow especially on darwin builds and takes +35minutes on the github runners.
+
+## Differences with official sharp-libvips
+
+### Supported platforms
+
+We have removed some of the supported platforms and architectures from the official `sharp-libvips`, as Session is not planning to support them. Some of those remove are:
+
+- linux-musl (glibc alternative)
+- linux-armv6
+- linux-ppc64le
+- linux-riscv64
+- linux-s390x
+- win32 that are not for x64
+
+What we do support is:
+
+- darwin-x64
+- darwin-arm64
+- linux-x64
+- linux-arm64v8
+- win32-x64
+
+**Note:**
+
+- **linux-sse2-x64** is not supported/tested yet, but we have added it to our fork.
+- **linux-x64**, we have added a way to build `libvips` statically (i.e. `.a`) and kept the `.so` files. The `sharp.node` build is then allowed to pick one of the two depending on the platform when it gets built.
+
+## Publishing
+
+Our custom build is not publishing to npm, but only to [github releases](https://github.com/session-foundation/sharp-libvips/releases/).
+
+Each build on each platform is uploaded separately and includes the files needed to build the `sharp.node` module for that specific platform.
+There is no top level `sharp-libvips` package like we have of `sharp`.
+So after a build through the CI, a new release is created on github, and that release contains a bunch .tar.gz that the prebuilt binaries.
+So the release is not a npm package, but a set of `npm pack` .tar.gz files, uploaded to github as a release.
+
+## Build a new release of sharp-libvips
+
+If you do make changes to `sharp-libvips`, you'll need to build a new release.
+The version doesn't have to match the official `sharp-libvips` version, but it should be a new version.
+
+To build a new release through the CI, all you need to do is to
+
+- remove the existing release on github for that version (Note: make sure no official release of session is using it, otherwise create a new version)
+- change the release to be built by searching and replacing the current version in this project. For instance, search and replace `1.2.4` with `1.2.5`.
+- remove the tag locally and remotely, and force push the changes (including the new tag)
+- `git tag -d v1.2.5; git tag v1.2.5 && git push foundation -f && git push foundation -f tags/v1.2.5`
+
+Note: the build needs a tag to generate a release, hence why you should delete and force push the tag each time you are iterating.
+
+## Integrate that change in a new release of sharp
+
+Once you have a new release of `sharp-libvips`, you can integrate it in a new release of `sharp`.
+To see the steps for this, check the `README.session.md` in our own `session-foundation/sharp` repository.