Add release keystore for APK signing

- Added rr3-release.keystore (RSA 2048-bit, valid 27 years)
- Alias: rr3key, Password: rr3community
- Added KEYSTORE-README.md with usage instructions
- Required for signing all future releases
- Community keystore shared publicly for team use

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

KEYSTORE-README.md

@@ -0,0 +1,60 @@
+# RR3 Release Keystore
+
+This keystore is used to sign all RR3 offline mod releases.
+
+## Keystore Details
+
+- **File:** rr3-release.keystore
+- **Alias:** rr3key
+- **Password:** rr3community
+- **Key Algorithm:** RSA 2048-bit
+- **Validity:** 10,000 days (~27 years)
+- **Created:** 2026-02-18
+
+## Usage
+
+### Signing an APK
+
+```bash
+# Step 1: Zipalign
+zipalign -v -p 4 input.apk aligned.apk
+
+# Step 2: Sign
+apksigner sign --ks rr3-release.keystore \
+  --ks-key-alias rr3key \
+  --ks-pass pass:rr3community \
+  --key-pass pass:rr3community \
+  --out signed.apk \
+  aligned.apk
+
+# Step 3: Verify
+apksigner verify -v signed.apk
+```
+
+### Windows PowerShell
+
+```powershell
+# Zipalign
+& "path\to\zipalign.exe" -v -p 4 input.apk aligned.apk
+
+# Sign
+& "path\to\apksigner.bat" sign --ks rr3-release.keystore --ks-key-alias rr3key --ks-pass pass:rr3community --key-pass pass:rr3community --out signed.apk aligned.apk
+
+# Verify
+& "path\to\apksigner.bat" verify -v signed.apk
+```
+
+## Important Notes
+
+⚠️ **Keep this keystore safe!** All future updates must be signed with the same key, otherwise users will have to uninstall the old version before installing updates.
+
+⚠️ **This is a community keystore** with a simple password for ease of use. It's shared publicly in this repository.
+
+## Certificate Information
+
+- **Owner:** CN=RR3Community, OU=Modding, O=Community, L=Unknown, ST=Unknown, C=US
+- **Valid until:** 02/18/2026 19:16:33.AddDays(10000).ToString('yyyy-MM-dd')
+
+## Files in This Release
+
+All APKs should be signed with this keystore before distribution.