add CHANGELOG.md

CHANGELOG.md

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+# [v1.0.0]  2024-04-02
+
+## Added
+
+- Basic four arithmetic operations: addition, subtraction, multiplication, and division.
+- Power function.

README.md

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+# C64-multi-functional-calculator
+
+Multi-functional, text-based RPN (Reverse Polish Notation) calculator designed specifically for the Commodore 64. This project brings a robust set of mathematical capabilities to the iconic Commodore 64, leveraging its unique architecture to deliver a fast, efficient, and user-friendly calculator experience
+
+## Description
+
+- addition '+'
+- subtraction '-'
+- division '/'
+- multiplication '*'
+- power '↑'
+
+Additionally, it accommodates floating-point numbers up to nine digits in both decimal (e.g., 3.14) and scientific (e.g., 8.9e-5) notations.
+
+## App development setup
+
+### Prerequisites
+
+- Installed [cc65](https://www.cc65.org/)
+
+### Build binary app
+
+- Clone the project:
+
+```bash
+git clone https://github.com/medvecky/C64-multi-functional-calculator.git
+```
+
+- Navigate to the project directory.
+- Execute the following command:
+
+```bash
+make multicalc
+```
+
+The calculator binary, named multicalc.prg, is located in the /bin directory.
+
+
+## Operating Manual
+
+### Simple Operations in RPN Format
+
+```bash
+# 9 * 4
+9 4 * <return>
+# 9 + 4
+9 4 + <return>
+# 9  / 4 
+9 4 / <return>
+# 9 ^ 4
+9 4 ↑ <return>
+```
+
+### Complex Operations in RPN Format
+
+```bash
+#√( 2 * 300.51 / 9.8 )
+300.51 2 * 9.8 / 0.5 ↑
+```