Add README file

Author: siarhiejkresik

README.md

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+# pycalc
+
+[![Build Status](https://travis-ci.org/siarhiejkresik/Epam-2019-Python-Homework.svg?branch=master)](https://travis-ci.org/siarhiejkresik/Epam-2019-Python-Homework)  
+Python Programming Language Foundation Hometask (EPAM, 2019).
+For task description see [link](https://github.com/siarhiejkresik/Epam-2019-Python-Homework/tree/master/final_task).
+
+`pycalc` is a command-line calculator implemented in pure Python 3 using Top Down Operator Precedence parsing algorithm (Pratt parser). It receives mathematical expression string as an argument and prints evaluated result.
+
+## Features
+
+`pycalc` supports:
+
+- arithmetic operations (`+`, `-`, `*`, `/`, `//`, `%`, `^` (`^` is a power));
+- comparison operations (`<`, `<=`, `==`, `!=`, `>=`, `>`);
+- 2 built-in python functions: `abs` and `round`;
+- all functions and constants from standard python module `math` (trigonometry, logarithms, etc.);
+- functions and constants from the modules provided with `-m` or `--use-modules` command-line option;
+- exit with non-zero exit code on errors.
+
+## How to install
+
+1. `git clone <repository_url>`
+2. `cd <repository_name>/final_task/`
+3. `pip3 install --user .` or `sudo -H pip3 install .`
+
+## Examples
+
+### Command line interface:
+
+```shell
+$ pycalc --help
+usage: pycalc [-h] EXPRESSION [-m MODULE [MODULE ...]]
+
+Pure-python command-line calculator.
+
+positional arguments:
+  EXPRESSION            expression string to evaluate
+
+optional arguments:
+  -h, --help            show this help message and exit
+  -m MODULE [MODULE ...], --use-modules MODULE [MODULE ...]
+                        additional modules to use
+```
+
+### Calculation:
+
+```shell
+$ pycalc '2+2*2'
+6
+
+$ pycalc '2+sin(pi)^(2-cos(e))'
+2.0
+```
+
+```shell
+$ pycalc '5+3<=1'
+False
+```
+
+```shell
+$ pycalc 'e + pi + tau'
+12.143059789228424
+
+$ pycalc '1 + inf'
+inf
+
+$ pycalc '1 - inf'
+-inf
+
+$ pycalc 'inf - inf'
+nan
+
+$ pycalc 'nan == nan'
+False
+```
+
+### Errors:
+
+```shell
+$ pycalc '15*(25+1'
+ERROR: syntax error
+15*(25+1
+        ^
+$ pycalc 'func'
+ERROR: syntax error
+func
+^
+$ pycalc '10 + 1/0 -3'
+ERROR: division by zero
+10 + 1/0 -3
+      ^
+$ pycalc '1 + sin(1,2) - 2'
+ERROR: sin() takes exactly one argument (2 given)
+1 + sin(1,2) - 2
+    ^
+$ pycalc '10^10^10'
+ERROR: math range error
+10^10^10
+  ^
+$ pycalc  '(-1)^0.5'
+ERROR: math domain error
+(-1)^0.5
+    ^
+$ pycalc ''
+ERROR: empty expression provided
+
+$ pycalc '1514' -m fake calendar nonexistent time
+ERROR: no module(s) named fake, nonexistent
+```
+
+### Additional modules:
+
+```python
+# my_module.py
+def sin(number):
+    return 42
+```
+
+```shell
+$ pycalc 'sin(pi/2)'
+1.0
+$ pycalc 'sin(pi/2)' -m my_module
+42
+$ pycalc 'THURSDAY' -m calendar
+3
+$ pycalc 'sin(pi/2) - THURSDAY * 10' -m my_module calendar
+12
+```
+
+## References
+
+- https://en.wikipedia.org/wiki/Pratt_parser
+- https://tdop.github.io/
+- http://www.oilshell.org/blog/2017/03/31.html
+- https://engineering.desmos.com/articles/pratt-parser