Added a README.md

Author: c0dn

README.md

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+# Growtopia Items.dat decoder
+
+This program decodes items.dat file from Growtopia game and outputs the decoded data in JSON format. \
+**Max supported version: 14**
+
+## Usage
+
+```commandline
+Growtopia items.dat decoder 1.0
+This program decodes the items.dat file found in growtopia game files. Outputs the information in JSON format
+
+USAGE:
+    growtopia_item_decode.exe [OPTIONS]
+
+FLAGS:
+    -h, --help       Prints help information
+    -V, --version    Prints version information
+
+OPTIONS:
+    -f, --file <file>        The items.dat file path (default: items.dat)
+    -o, --output <output>    The file name to write the data to (default: items.json)
+```