Add files via upload (#930)

Update README.rst

Add description of required CMake variables.

Co-authored-by: Andras Lasso <lasso@queensu.ca>

README.rst

@@ -1,11 +1,23 @@
-Common Toolkit
-==============
-
-.. image:: https://circleci.com/gh/commontk/CTK.png?style=shield
-    :target: https://circleci.com/gh/commontk/CTK
-
-The Common Toolkit is a community effort to provide support code for medical image analysis,
-surgical navigation, and related projects.
-
-See http://commontk.org
-
+Common Toolkit
+==============
+
+.. image:: https://circleci.com/gh/commontk/CTK.png?style=shield
+    :target: https://circleci.com/gh/commontk/CTK
+
+The Common Toolkit is a community effort to provide support code for medical image analysis,
+surgical navigation, and related projects.
+
+See http://commontk.org
+
+Build Instructions
+==================
+
+Configure the project using CMake.
+
+For Qt5, specify the followings:
+- `CTK_QT_VERSION`: 5
+- `QT5_DIR`: C:\Qt\5.15.0\msvc2019_64\lib\cmake\Qt5 (or something similar, depending on operating system)
+- `VTK_MODULE_ENABLE_VTK_GUISupportQt`: YES (for enabling VTK widgets)
+- `VTK_MODULE_ENABLE_VTK_ViewsQt`: YES (for enabling VTK view widgets)
+
+Note: make sure your built toolchain version is compatible with the chosen Qt version. For example if trying to build with Qt-5.12 and Microsoft Visual Studio 2019, then build will fail with the error `error LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl QLinkedListData::QLinkedListData(void)"`. The solution is to either change the toolset version to an earlier one (e.g., Visual Studio 2017) or upgrade Qt (e.g., to Qt-5.15.0).