add build.md

BUILD.md

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+## How to build and test ydb-python-sdk
+
+This document has detailed instructions on how to build ydb-python-sdk from source and run style and unit tests.
+
+### Pre-requisites
+
+- Install [Docker](https://docs.docker.com/engine/install/).
+- Install [Python 3.8](https://www.python.org/downloads/release/python-38/)
+- Install [pip](https://pip.pypa.io/en/latest/installation/)
+- Install [Tox](https://tox.wiki/en/latest/install.html)
+
+### Clone the repository
+
+```sh
+git clone https://github.com/ydb-platform/ydb-python-sdk
+```
+
+### Run style tests
+
+Use the command below to prepare `virtualenv` and to run style tests using `flake8`.
+
+```sh
+tox -estyle
+```
+
+### Run formatting checks
+
+Use the command below to prepare `virtualenv` and to run style tests using `black`.
+See [documentation](https://black.readthedocs.io/en/stable/) about Black project.
+
+```sh
+tox -eblack
+```
+
+To automatically format code using the `black` formatting style, use the command below.
+
+```sh
+tox -eblack-format
+```
+
+### Run unit tests
+
+Use the command below to run unit tests.
+
+```sh
+tox -epy310
+```