docs: Document how we created the experience-cs fork

As part of the Google CS First handover, Google were keen for us to be be using
a more up-to-date release of blockly (v12 beta). They pointed us towards a
Google fork `gonfunko/modern-blockly` [1] (maintained by Aaron Dodson). This
fork contained a number of improvements / fixes from Google, however did not
depend on a more up-to-date release of blockly. The team later discovered that
there was a `beta` branch [2] on the scratchfoundation repo which incorporated
most of Aaron Dodson's changes, but also updated the dependencies on
`scratch-blocks` (v2 beta) and `scratch-vm` to pull in the more up-to-date
release of blockly.

The `beta` branch was based off an old release (4.1.0.beta1) [3].  Unfortunately
none of these changes have made it upstream into a stable release of
scratch-gui.

After discovering some errors which were preventing the scratch editor from
loading on iOS, we have ended updating our fork to be based off of the latest v4
release of scratch-gui (v4.1.7)[4] from November 2024.

We were keen to keep Google happy, and fix these iOS issues, therefore we have
created a new base branch `experience-cs-base`[5] to replace the `beta` branch
initially provided by Google.

In order to avoid future headaches/confusion here are the steps used to create
our experience-cs-base branch:

```
git clone git@github.com:RaspberryPiFoundation/scratch-gui.git

git remote add mit https://github.com/scratchfoundation/scratch-gui.git
git fetch mit

git checkout mit/beta -b experience-cs-base
git rebase -i v4.1.7

** Remove final 3 commits which tweak the version number (unnecessary merge conflicts)
* pick f57add0 chore(release): 4.1.0-hotfix.1 [skip ci]
* pick e5cc322 fix(release): release beta branch under beta label(?)
* pick a1c4a87 chore(release): 4.1.0-beta.1 [skip ci]

** Resolve merge conflicts in package.json:
* "scratch-blocks": "^2.0.0-beta",
* "scratch-l10n": "^4.0.0",
git checkout HEAD package-lock.json
npm install
git rebase --continue

git push origin HEAD
```

[1] https://github.com/gonfunko/scratch-gui/commits/modern-blockly/
[2] https://github.com/scratchfoundation/scratch-gui/commits/beta
[3] https://github.com/scratchfoundation/scratch-gui/commits/v4.1.0-beta.1/
[4] https://github.com/scratchfoundation/scratch-gui/commits/v4.1.7
[5] https://github.com/RaspberryPiFoundation/scratch-gui/commits/experience-cs-base

Author: sebjacobs

README.md

@@ -6,6 +6,63 @@ To open the current build in your browser on Github Pages:
 
 https://scratchfoundation.github.io/scratch-gui/
 
+## How did we create the experience-cs fork of scratch-gui?
+
+As part of the Google CS First handover, Google were keen for us to be be using
+a more up-to-date release of blockly (v12 beta). They pointed us towards a
+Google fork `gonfunko/modern-blockly` [1] (maintained by Aaron Dodson). This
+fork contained a number of improvements / fixes from Google, however did not
+depend on a more up-to-date release of blockly. The team later discovered that
+there was a `beta` branch [2] on the scratchfoundation repo which incorporated
+most of Aaron Dodson's changes, but also updated the dependencies on
+`scratch-blocks` and `scratch-vm` to pull in the more up-to-date release of
+blockly.
+
+The `beta` branch was based off an old release (4.1.0.beta1) [3]. Unfortunately
+none of these changes have made it upstream into a stable release of
+scratch-gui.
+
+After discovering some errors which were preventing the scratch editor from
+loading on iOS, we have ended updating our fork to be based off of the latest v4
+release of scratch-gui (v4.1.7)[4] from November 2024.
+
+We were keen to keep Google happy, and fix these iOS issues, therefore we have
+created a new base branch `experience-cs-base`[5] to replace the `beta` branch
+initially provided by Google.
+
+In order to avoid future headaches/confusion here are the steps used to create
+our experience-cs-base branch:
+
+```
+git clone git@github.com:RaspberryPiFoundation/scratch-gui.git
+
+git remote add mit https://github.com/scratchfoundation/scratch-gui.git
+git fetch mit
+
+git checkout mit/beta -b experience-cs-base
+git rebase -i v4.1.7
+
+** Remove final 3 commits which tweak the version number (unnecessary merge conflicts)
+* pick f57add04a chore(release): 4.1.0-hotfix.1 [skip ci]
+* pick e5cc322e8 fix(release): release beta branch under beta label(?)
+* pick a1c4a873b chore(release): 4.1.0-beta.1 [skip ci]
+
+** Resolve merge conflicts in package.json:
+* "scratch-blocks": "^2.0.0-beta",
+* "scratch-l10n": "^4.0.0",
+git checkout HEAD package-lock.json
+npm install
+git rebase --continue
+
+git push origin HEAD
+```
+
+[1] https://github.com/gonfunko/scratch-gui/commits/modern-blockly/
+[2] https://github.com/scratchfoundation/scratch-gui/commits/beta
+[3] https://github.com/scratchfoundation/scratch-gui/commits/v4.1.0-beta.1/
+[4] https://github.com/scratchfoundation/scratch-gui/commits/v4.1.7
+[5] https://github.com/RaspberryPiFoundation/scratch-gui/commits/experience-cs-base
+
 ## Installation
 
 This requires you to have Git and Node.js installed.