gms: add readme

README.md

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+# vendor_gms
+
+## Basic usage
+
+- If you just wanna use this repository, then you just need to include `products/gms.mk`
+- This is as simple as: `$(call inherit-product-if-exists, vendor/gms/products/gms.mk)`
+
+## Advanced usage
+
+### Initial setup
+- Install `xmlstarlet`
+  - `sudo apt install xmlstarlet`, `sudo pacman -S xmlstarlet`, etc. depending on your distribution.
+- Install `apktool`, make sure its available in PATH
+  - https://ibotpeaches.github.io/Apktool/install/
+  - Run `apktool if framework-res.apk` to install framework for apktool this is necessary for proper extracting
+    (generally found in /system/framework/)
+
+### Extracting / updating extracted overlays
+
+Basically the same as regular extract-files
+
+```sh
+./extract-files.sh /path/to/a/dump
+```
+
+### Removing certain overlays
+
+- If certain overlays are not required, you can simply add them to `exclude-tag.txt`, those will be removed from the extracted overlays.
+
+### Miscellaneous information
+
+- The main changes here are the addition of `apktool` and `xmlstarlet`, to allow us to extract the necessary overlays from some RROs.
+- Since the process does take a while, to speed it up, all extraction is done in the background. This might not be suitable for weaker systems if there is a very high number of overlays to be extracted.