[docs] add instructions to build older commits (#3826)

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3826

docs/README.md

@@ -16,6 +16,7 @@ This contains documentation created by developers. This contains build instructi
 - **[Dynarmic](./dynarmic)**
 - **[Cross compilation](./CrossCompile.md)**
 - **[Driver Bugs](./DriverBugs.md)**
+- **[Building Older Commits](./build/OlderCommits.md)**
 
 ## Policies
 

docs/build/OlderCommits.md

@@ -0,0 +1,40 @@
+# Building Older Commits
+
+Bisecting and debugging older versions of Eden can be difficult, as many of our submodules have been deleted or removed. However, work has been done to make this process as simple as possible for users.
+
+## Script
+
+Copy the following script and store it in `fix.sh`:
+
+```sh
+#!/bin/sh -e
+
+git -C externals/discord-rpc checkout 0d8b2d6a37c6e47d62b37caa14708bf747c883bb
+git add externals/discord-rpc
+
+git -C externals/dynarmic checkout 05b7ba50588d1004e23ef91f1bda8be234be68f4
+git add externals/dynarmic
+
+git -C externals/mbedtls checkout ce4f81f4a926a0e0dcadd0128e016baba416e8ea
+git add externals/mbedtls
+
+git -C externals/oboe checkout e4f06f2143eb0173bf4a2bd15aae5e8cc3179405
+git add externals/oboe
+
+git -C externals/sirit checkout b870b062998244231a4f08004d3b25151732c5c5
+git add externals/sirit
+```
+
+Then, run `chmod +x fix.sh`
+
+## Submodules
+
+To check out submodules successfully, use this order of operations:
+
+```sh
+git submodule update --init --recursive --depth 1 --jobs 8 --progress
+./fix.sh
+git submodule update --init --recursive --depth 1 --jobs 8 --progress
+```
+
+And you should be good to go! If you check out a different commit that changes submodule commits, run the above command list again.