Update README to add Cmake information

Adds some basic build & installation information.
Adds the list of special vars available after find_package.
Also adds a small note on how to use precompiled headers.

Author: sum01

README.md

@@ -512,6 +512,44 @@ cli.set_compress(true);
 res = cli.Post("/resource/foo", "...", "text/plain");
 ```
 
+Building & Installation (Cmake)
+-------------------------------
+
+After installation with Cmake, a `find_package(httplib)` is available.  
+This creates a `httplib::httplib` target (if found).  
+It can be linked like so: 
+```cmake
+target_link_libraries(your_exe httplib::httplib)
+```
+
+The following will build & install for later use.
+
+Linux/macOS
+```shell
+mkdir -p build
+cd build
+cmake -DCMAKE_BUILD_TYPE=Release ..
+sudo cmake --build . --target install
+```
+Windows
+```cmd
+mkdir build
+cd build
+cmake ..
+runas /user:Administrator "cmake --build . --config Release --target install"
+```
+
+These three variables are available after you run `find_package(httplib)`
+* `HTTPLIB_HEADER_PATH` - this is the full path to the installed header.
+* `HTTPLIB_IS_USING_OPENSSL` - a bool for if OpenSSL support is enabled.
+* `HTTPLIB_IS_USING_ZLIB` - a bool for if ZLIB support is enabled.
+
+Want to use precompiled headers (Cmake feature since v3.16)?  
+It's as simple as doing the following (before linking):
+```cmake
+target_precompile_headers(httplib::httplib INTERFACE "${HTTPLIB_HEADER_PATH}")
+```
+
 Split httplib.h into .h and .cc
 -------------------------------