doc: add build prerequisites

[ci skip]

Documentation/Building.md

@@ -1,11 +1,87 @@
 # Building and installing Libevent
+
 ### Jump to:
+
+- [Prerequisites](#Prerequisites)
 - [Autoconf](#autoconf)
   - [Flags](#autoconf-flags)
 - [Building on Windows](#building-on-windows)
 - [Building on Unix (With CMake)](#building-on-unix-cmake)
 - [CMake Variables](#cmake-variables)
 
+## Prerequisites
+
+### Linux deb-like (ubuntu/debian/...)
+
+Just install tools using your preferred package manager if you build using autotools:
+
+```sh
+sudo apt-get install automake autoconf libtool pkg-config
+```
+
+or build using cmake:
+
+```sh
+sudo apt-get install cmake
+```
+
+Doxygen is a tool for generating documentation. Git is used to fetch the package version.
+Install them if needed:
+
+```
+sudo apt-get install doxygen git
+```
+
+libevent has encryption layer, you need openssl or mbedTLS for it, you can
+install one of this using:
+
+```sh
+sudo apt-get install libssl-dev libmbedtls-dev
+```
+
+To support multithreaded environments, libpthread is a must, and it already exists in the system.
+
+To run the tests, you should install zlib:
+
+```sh
+sudo apt-get install zlib1g-dev
+```
+
+Finally, python interpreter should be installed if you want to run regression tests.
+
+### MacOS
+
+On MacOS you can use `brew` to manage packages.
+
+The installation process on MacOS is roughly the same as on Linux,
+the difference is installation of openssl and zlib:
+
+```sh
+brew install openssl zlib
+```
+
+### Windows
+
+To install it, there are two choices: installer and zip file.
+
+If using zip file, you should set the PATH variable in the Environment
+Variables for your User to include the installation path of cmake.
+
+Install Visual Studio which is the true compiler that will be used.
+
+Install OpenSSL to support for encryption, then add the installation path into the PATH variable in the Environment Variables,
+or set OPENSSL_ROOT_DIR in command prompt:
+
+```sh
+set "OPENSSL_ROOT_DIR=C:\path\to\OpenSSL"
+```
+
+or add `OPENSSL_ROOT_DIR` definition to the cmake command:
+
+```sh
+cmake -DOPENSSL_ROOT_DIR=C:/path/to/OpenSSL ...
+```
+
 ## Autoconf
 
 ```