General: Added readme.md

readme.md

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+## About
+
+Python API for opXRD database. See more at https://xrd.aimat.science
+
+## Setup
+
+### System level requirements
+The data file import functionalities provided by the xrdpattern requirement are largely built on the C++ library  [xylib](https://github.com/wojdyr/xylib) so beyond a standard python install this library also requires:
+- A C++ compiler and standard library
+- A C++ to python bridge (Swig)
+```
+sudo apt install build-essential	# GNU C++ compiler
+sudo apt install libboost-all-dev       # C++ libraries provided by Boost
+sudo apt install swig 			# python -> C++ bridge
+```
+
+For Windows >10 you may try the following in PowerShell (requires [Chocolatey](https://chocolatey.org/)):
+```
+choco install mingw  # C++ compiler and std library
+choco install swig   # python -> C++ bridge
+```
+
+### Python library
+Once the system level requirements are installed, the library can be installed using pip:
+```
+pip install opxrd
+```