Add README.

README.md

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+# screepsxx
+
+**screepsxx** is a C++ library for [Screeps](https://screeps.com) API.
+It provides wrappers for the majority of classes in [Screeps API](https://docs.screeps.com/api).
+
+## Requirements
+* [Emscripten](https://emscripten.org) toolchain
+* [CMake](https://cmake.org) 3.16+
+* [Ninja](https://ninja-build.org) build system
+* (Optional) Python 3 interpreter for artifacts uploading
+
+## Building
+```shell
+git clone https://github.com/UltraCoderRU/screepsxx.git
+cd screepsxx
+mkdir build
+cd build
+cmake -G Ninja -DCMAKE_TOOLCHAIN_FILE=<path_to_emsdk>/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_BUILD_TYPE=Release ..
+cmake --build .
+# Add -DSCREEPSXX_BUILD_EXAMPLE=ON to build example application (see 'example' directory)
+```
+
+## Using library in CMake project
+The simplest way to use screepsxx library is to add it to your project with `add_subdirectory()`:
+```cmake
+cmake_minimum_required(VERSION 3.16)
+
+project(MyProject CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -s STRICT=0 -s ASSERTIONS=0 -s ALLOW_MEMORY_GROWTH=1 -s ENVIRONMENT=shell -s MALLOC=emmalloc --cache ${CMAKE_BINARY_DIR}/cache")
+
+add_subdirectory(screepsxx)
+
+add_executable(myapp loop.cpp)
+target_link_libraries(myapp screepsxx)
+target_link_options(myapp PUBLIC -sMODULARIZE=1 --no-entry --bind)
+```
+
+Compiled WASM-module (`myapp.wasm`) and corresponding JavaScript module (`myapp.js`) will appear in your build directory (`CMAKE_BINARY_DIR`).
+To use them inside Screeps, you need two additional JavaScript modules located in `js` directory.
+Put all four files into one directory and upload them to Screeps server.
+You can use Python script `tools\upload.py`, see `example/CMakeLists.txt` for details.