update docs

UPDATES.md

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+# Updating an existing modpack
+
+## Behaviour
+
+TODO: use clear terminology
+- `packstate`
+- `old packstate`
+- `new packstate`
+- `packstate files/hashes`
+
+For an updated to be executed, the program performs the following steps:
+
+1. Read state information of old modpack version from `packstate.json` file.
+    * Download the mrpack archive of the new modpack version if required.
+2. Generate state information of new modpack version from mrpack archive file.
+3. If any version difference of dependencies exists, cancel the update.
+    * (`minecraft`, `fabric-loader`, `quilt-loader`, `forge`, `neoforge`)
+    * TODO: flag for continuing anyway
+    * TODO: actually also keep track of and update the dependencies
+4. For all files not changed by the update:
+    * If the file is stored inside the `mods` directory and
+    * the file is present in the old modpack version and
+    * the file is not present in the new modpack version, then
+    * create a backup of the file and delete the file.
+5. For all files changed by the update:
+    * If the file is not stored inside the `mods` directory and
+    * the file hash is not equal to the hash in the old modpack version and
+    * the file hash is not equal to the hash in the new modpack version, then
+    * create a backup of the file and overwrite the file.
+
+TODO: is this actually a sane behaviour for updates? mock some scenarios
+TODO: correctly implemented the planned behaviour and write tests
+
+## packstate.json
+
+To keep track of changes, mrpack-install creates a `packstate.json` file to store information about the state of a
+modpack installation.
+
+#### Example
+
+```json
+{
+  "slug": "my-cool-pack",
+  "project-id": "TDM2I9Mu",
+  "version": "1.3.2",
+  "version-id": "TpN85eu2",
+  "dependencies": {
+    "minecraft": "1.19.2",
+    "fabric-loader": "0.14.10"
+  },
+  "hashes": {
+    "INFOS.txt": {
+      "sha1": "8548257fd702d58e02bd58f6bcc8affbb96a5d38",
+      "sha512": "cf83e1357eefb8bdf1542850d66d8007d621e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec3f63b931bd47417a81a538327af927da3e"
+    },
+    "mods/ultra-blocks-1.2.3+mc1.19.2.jar": {
+      "sha1": "df8e2fbabdbeea9c5acea5b1178ad3bb3d75fcf7",
+      "sha512": "9884fcf6e1f10c1a6e85fd41515bc2e05390c160dd9c70844d1814a0085dd42c850129b5bf175f6d89ca40b86fbe6407dc3b9a48fb8f393fbd68eafa3b9c5b8c"
+    }
+  }
+}
+```