add initial change set docs

packages/lix-sdk/docs/20-concepts/20-change.md

@@ -2,7 +2,7 @@
 
 ## Purpose 
 
-Store information of what changed in a [file](./10-file.md). 
+Stores a change for an [entity](./21-entity.md) along with a [snapshot](./22-snapshot.md).
 
 ## Description
 

packages/lix-sdk/docs/20-concepts/21-entity.md

@@ -0,0 +1,25 @@
+# Entity
+
+## Purpose 
+
+Defines what is being changed. 
+
+## Description
+
+An entity is the subject of a change. It can be a file, a database table, a configuration, or any other data structure.
+
+## Examples
+
+### CSV File Format
+
+An examplary csv plugin tracks changes for the rows in a csv file. Hence, the entities are `row`. 
+
+### Inlang File Format
+
+An examplary inlang plugin tracks changes for bundles, messages and variants. Hence, the entities are `bundle`, `message`, `variant`.
+
+### Markdown File Format
+
+An examplary markdown plugin tracks changes for the lines in a markdown file. Hence, the entities are `line`.
+
+

packages/lix-sdk/docs/20-concepts/22-snapshot.md

@@ -0,0 +1,44 @@
+# Snapshot
+
+## Purpose 
+
+Stores the state of a [change](./20-change.md).
+
+## Description
+
+The snapshot is used to re-apply changes, revert changes, compare changes, and more.
+
+## Technical Details
+
+- Snapshot's are [content adressable](https://en.wikipedia.org/wiki/Content-addressable_storage) to save storage. 
+
+- IDs are generated by hashing the content of the snapshot.
+
+- If a snapshot is empty (e.g., a file is empty), the snapshot is has a special ID `empty-content`.
+
+## Examples
+
+### CSV File Format
+
+An examplary csv plugin tracks changes in rows. Hence, the snapshot is a row.
+
+Suppose the following table is a CSV file:
+
+| id | name | age |
+|----|------|-----|
+| 1  | John | 25  |
+
+The snapshot of the row with `id=1` is:
+
+`1, John, 25`
+
+Someone updates John's age from 25 to 26. 
+
+| id | name | age |
+|----|------|-----|
+| 1  | John | 26  |
+
+The snapshot of the row with `id=1` after the change is:
+
+`1, John, 26`
+

packages/lix-sdk/docs/20-concepts/50-change-set.md

@@ -0,0 +1,79 @@
+# Change Set
+
+## Purpose
+
+Group related changes.
+
+## Description  
+
+Changes in a set are treated as a single unit. They are merged, applied, described (commentied) and undone together. 
+
+## Graph 
+
+Changes D and B are in a set.
+
+```mermaid
+graph TB
+    subgraph Set
+    direction TB
+    D["Change D"]
+    B["Change B"]
+    end
+
+    E["Change E"]
+    X["Change X"]
+    A["Change A"]
+    F["Change F"]
+
+    X --> D
+    B --> A
+    F --> B
+    D --> E
+```
+
+## Examples
+
+### Discussions/Commenting
+
+A [discussion](./90-discussion.md) points to a change set. A change set can contain 1 or many changes. 
+
+### Inventory of a deparment store
+
+A department store sells three types of shoes: **Shoe A**, **Shoe B**, and **Shoe C**. After each day, the inventory manager updates the stock levels and re-stocking dates in a CSV file.
+
+#### Initial Table
+
+| Product  | In Stock | Price |
+|----------|----------|-------------------------------|
+| Shoe A  | 120      | $89                    |
+| Shoe B  | 150      | $124                    |
+| Shoe C  | 200      | $78                    |
+
+#### Changes
+
+1. Change H - Update stock of **Shoe A** from 120 to 80 units.
+
+2. Change L - Update stock of **Shoe B** from 150 to 100 units.
+
+3. Change R - Update price of **Shoe C** from $78 to $89.
+
+#### Change Sets
+
+Both inventory stock changes are related because they reflect the stock level after 2022-09-01. If someone looks at the stock levels on 2022-09-01, they should see both changes.
+
+However, the price change the manager made right after updating the stock levels is an independent change, and therefore, the store manager decided to not group it with the stock changes.
+
+```plaintext
+Change Set 1
+
+- Comment: Update Stock Levels - date 2022-09-01
+- Changes: [Change H, Change L]
+```
+
+```plaintext
+Change Set 2
+
+- Comment: Increased the price due to high demand  
+- Changes: [Change R]
+```
+