add documentation for mod migration

docs/migration.md

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+# Migration guides
+
+## Generic migration
+Migration for the typical mod which doesn't usy anything special but minecraft forge and some library dependencies. 
+For core plugin, mixins, shadowing, access transformes, asm or etc. you'll need to do some extra steps. 
+If they are missing in this document - we will gladly receive your suggestions/contribution. 
+
+1. Copy and replace all files from template to your repository, but: // TODO: (maybe we should have a task in CI which attaches zip with all files but these, called "migration-bundle.zip" or smth)
+   1. `build.gradle`
+   2. `src`
+   3. `LICENSE`
+   4. `README.md`
+   5. `build.gradle(.kts)`
+2. Copy all repositories from your `build.gradle(.kts)` to `repositories.gradle`
+3. Copy all dependencies from your `build.gradle(.kts)` to `dependecies.gradle`
+4. replace your `build.gradle(.kts)` with `build.gradle` from template. In case you have written some custom tasks/configurations not present in the template - you'll best to move them to separate file and link it in this, like for example it does this with `dependencies.gradle`
+5. Adapt `gradle.properties` to your mod
+6. Ensure `src/main/resources/mcmod.info` contains `${modId}`, `${modName}`. `${modVersion}` and `${minecraftVersion}`
+7. Run `./gradlew clean setupDecompWorkspace`
+
+## Mixin configuration
+For the reference checkout the [example mixin configuration branch](https://github.com/SinTh0r4s/ExampleMod1.7.10/tree/example-mixins) of the template.
+
+1. Extract mixins package and plugin configuration from `mixins.yourModId.json` to `gradle.properties`
+2. Implement MixinPlugin according to example from the reference
+3. Remove mixins.mymodid.json