docs: Add build description.

docs/build.md

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+
+## Dependencies
+
+Currently, it is necessary to use `clang` (due to `gcc` bug) to build VAST. On Linux it is also necessary to use `lld` at the moment.
+
+VAST uses `llvm-17` which can be obtained from the [repository](https://apt.llvm.org/) provided by LLVM.
+
+Before building (for Ubuntu) get all the necessary dependencies by running
+```
+apt-get install build-essential cmake ninja-build libstdc++-12-dev llvm-17 libmlir-17 libmlir-17-dev mlir-17-tools libclang-17-dev
+```
+or an equivalent command for your operating system of choice.
+
+## Instructions
+
+To configure project run `cmake` with following default options.
+In case `clang` isn't your default compiler prefix the command with `CC=clang CXX=clang++`.
+If you want to use system installed `llvm` and `mlir` (on Ubuntu) use:
+
+```
+cmake --preset ninja-multi-default \
+    --toolchain ./cmake/lld.toolchain.cmake \
+    -DCMAKE_PREFIX_PATH=/usr/lib/llvm-17/
+```
+
+To use a specific `llvm` provide `-DCMAKE_PREFIX_PATH=<llvm & mlir instalation paths>` option, where `CMAKE_PREFIX_PATH` points to directory containing `LLVMConfig.cmake` and `MLIRConfig.cmake`.
+
+Note: vast requires LLVM with RTTI enabled. Use `LLVM_ENABLE_RTTI=ON` if you build your own LLVM.
+
+
+Finally, build the project:
+
+```
+cmake --build --preset ninja-rel
+```
+
+Use `ninja-deb` preset for debug build.
+
+## Run
+
+To run mlir codegen of highlevel dialect use.
+
+```
+./builds/ninja-multi-default/tools/vast-front/Release/vast-front -vast-emit-mlir=<dialect> <input.c> -o <output.mlir>
+```
+
+Supported dialects are: `hl`, `ll`, `llvm`
+
+## Test
+
+```
+ctest --preset ninja-deb
+```