readme: add install instructions

README.md

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+# Browsercore
+
+## Build
+
+### Prerequisites
+
+Browsercore is written with [Zig](https://ziglang.org/) `0.10.1`. You have to
+install it with the right version in order to build the project.
+
+Browsercore also depends on
+[js-runtimelib](https://github.com/francisbouvier/jsruntime-lib/) and
+[lexbor](https://github.com/lexbor/lexbor) libs.
+
+To be able to build the v8 engine for js-runtimelib, you have to install some libs:
+
+For Debian/Ubuntu based Linux:
+```
+sudo apt install xz-utils \
+    python3 ca-certificates git \
+    pkg-config libglib2.0-dev \
+    cmake
+```
+
+For MacOS, you only need Python 3 and cmake.
+
+To be able to build lexbor, you need to install also `cmake`.
+
+### Install and build dependencies
+
+The project uses git submodule for dependencies.
+The `make install-submodule` will init and update the submodules in the `vendor/`
+directory.
+
+```
+make install-submodule
+```
+
+### Build lexbor
+
+The command `make install-lexbor` will build lexbor lib used by browsercore.
+```
+make install-lexbor
+```
+
+### Build jsruntime-lib
+
+The command `make install-jsruntime-dev` uses jsruntime-lib's `zig-v8` dependency to build v8 engine lib.
+Be aware the build task is very long and cpu consuming.
+
+Build v8 engine for debug/dev version, it creates
+`vendor/jsruntime-lib/vendor/v8/$ARCH/debug/libc_v8.a` file.
+
+```
+make install-jsruntime-dev
+```
+
+You should also build a release vesion of v8 with:
+
+```
+make install-jsruntime
+```
+
+### All in one build
+
+You can run `make intall` and `make install-dev` to install deps all in one.
+
+## Test
+
+You can test browsercore by running `make test`.