Add readme

README.md

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+# Basic calculator
+
+## Requirements
+
+Please install peg/leg manually or with Homebrew:
+```bash
+$ brew install peg
+```
+
+## Build
+
+```bash
+$ make
+```
+
+## Usage
+
+```bash
+$ echo "2+3" | ./calc
+```
+
+## Test
+
+```bash
+$ ./test.sh
+```