Substantial documentation improvements

docs/guide/migrating-to-epiphany.md

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+Rojo underwent a large refactor during most of 2018 to enable a bunch of new features and lay groundwork for lots more in 2019. As such, Rojo **0.5.x** projects are not compatible with Rojo **0.4.x** projects.
+
+[TOC]
+
+## Supporting Both 0.4.x and 0.5.x
+Rojo 0.5.x uses a different name for its project format. While 0.4.x used `rojo.json`, 0.5.x uses `default.project.json`, which allows them to coexist.
+
+If you aren't sure about upgrading or want to upgrade gradually, it's possible to keep both files in the same project without causing problems.
+
+## Upgrading Your Project File
+Project files in 0.5.x are more explicit and flexible than they were in 0.4.x. Project files can now describe models and plugins in addition to places.
+
+This new project file format also guards against two of the biggest pitfalls when writing a config file:
+
+* Using a service as a partition target directly, which often wiped away extra instances
+* Defining two partitions that overlapped, which made Rojo act unpredictably
+
+The biggest change is that the `partitions` field has been replaced with a new field, `tree`, that describes the entire hierarchy of your project from the top-down.
+
+A project for 0.4.x that syncs from the `src` directory into `ReplicatedStorage.Source` would look like this:
+
+```json
+{
+    "name": "Rojo 0.4.x Example",
+    "partitions": {
+        "path": "src",
+        "target": "ReplicatedStorage.Source"
+    }
+}
+```
+
+In 0.5.x, the project format is more explicit:
+
+```json
+{
+    "name": "Rojo 0.5.x Example",
+    "tree": {
+        "$className": "DataModel",
+        "ReplicatedStorage": {
+            "$className": "ReplicatedStorage",
+            "Source": {
+                "$path": "src"
+            }
+        }
+    }
+}
+```
+
+For each object in the tree, we define *metadata* and *children*.
+
+Metadata begins with a dollar sign (`$`), like `$className`. This is so that children and metadata can coexist without creating too many nested layers.
+
+All other values are considered children, where the key is the instance's name, and the value is an object, repeating the process.
+
+## Migrating Unknown Files
+If you used Rojo to sync in files as `StringValue` objects, you'll need to make sure those files end with the `txt` extension to preserve this in Rojo 0.5.x.
+
+Unknown files are now ignored in Rojo instead of being converted to `StringValue` objects.