Updates README cmake instruction for Windows.

Signed-off-by: Camila San <hello@camila.codes>

README.md

@@ -32,7 +32,7 @@ you want to install system wide you could use `/usr/local` or `/opt/nextcloud/`.
 ##### Linux
 
 ```
-$ cmake .. -DCMAKE_INSTALL_PREFIX=path-to-install-folder/ -DCMAKE_BUILD_TYPE=Debug -DNO_SHIBBOLETH=1
+$ cmake "-GVisual Studio 15 2017 Win64" .. -DCMAKE_INSTALL_PREFIX=path-to-install-folder/ -DCMAKE_BUILD_TYPE=Debug -DNO_SHIBBOLETH=1 -DPng2Ico_EXECUTABLE=/path-to-install-png2ico/png2ico.exe  -DQTKEYCHAIN_LIBRARY=/path-to-qt5keychain-folder/lib/qt5keychain.lib -DQTKEYCHAIN_INCLUDE_DIR=/path-to-qt5keychain-folder/include/qt5keychain/ -DOPENSSL_ROOT_DIR=/path-to-openssl-folder/ -DOPENSSL_INCLUDE_DIR=path-to-openssl-folder/include -DOPENSSL_LIBRARIES=path-to-openssl-folder/lib
 $ make install
 ```