update readme

Author: hks2002

README.md

@@ -32,6 +32,27 @@ cmake -B build -H.
 cd build 
 make -j
 ```
+#### Building options
+
+Trantor provides some building options, you can enable or disable them by setting the corresponding variables to `ON` or `OFF` in the cmake command line, cmake file etc...
+
+| Option name | Description | Default value |
+| :--- | :--- | :--- |
+| BUILD_SHARED_LIBS | Build Trantor as a shared lib |  OFF | 
+| BUILD_TESTING | Build tests |  OFF | 
+| BUILD_DOC | Build Doxygen documentation |  OFF | 
+| BUILD_DEPENDENCIES | Fetch and build dependencies, this allow you use latest or change dependencies version, if you don't want to use system package manager provided | OFF | 
+| TRANTOR_USE_SPDLOG | Allow using the spdlog logging library | ON | 
+| TRANTOR_USE_C-ARES | Allow using C-ARES |  OFF |
+| TRANTOR_TLS_PROVIDER | TLS provider for trantor. Valid options are 'none', 'openssl', 'botan-3', 'auto'.   | auto  | 
+
+> With option `BUILD_DEPENDENCIES` on, the related dependencies source will download and build automatically based on the options settings, this make develop more easily in `Windows`.
+>* Build `openssl` needs `perl` and build `botan` needs `python`.
+>* Passing `-DCMAKE_TOOLCHAIN_FILE="your-toolchain.cmake"` will turn off `BUILD_DEPENDENCIES`.
+
+> There are some scripts named start with `deps.*` for you to quickly install all dependencies also.
+>* If you want to use system installed package manager (such as: brew, apt, dnf) provided dependencies, please turn off option `BUILD_DEPENDENCIES`.
+>* If these scripts is not fit your system, you can turn on option `BUILD_DEPENDENCIES`.
 
 ## Licensing
 Trantor - A non-blocking I/O based TCP network library, using C++14.