Create README.md

README.md

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+# Math_REPL
+
+Simple interactive environment which calculate math expression, support functions and variables
+
+Supported grammar:
+
+function ::= fn-keyword fn-name { identifier } fn-operator expression
+
+fn-name ::= identifier
+
+fn-operator ::= '=>'
+
+fn-keyword ::= 'fn'
+
+expression ::= factor | expression operator expression
+
+factor ::= number | identifier | assignment | '(' expression ')' | function-call
+
+assignment ::= identifier '=' expression
+
+function-call ::= fn-name { expression }
+
+operator ::= '+' | '-' | '*' | '/' | '%'
+
+identifier ::= letter | '_' { identifier-char }
+
+identifier-char ::= '_' | letter | digit
+
+number ::= { digit } [ '.' digit { digit } ]
+
+letter ::= 'a' | 'b' | ... | 'y' | 'z' | 'A' | 'B' | ... | 'Y' | 'Z'
+
+digit ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
+
+Application screenshots: 
+
+![image](https://user-images.githubusercontent.com/66470614/176257090-8880c5b1-be36-443a-8d56-bba67945e996.png)