Mention tdlib version requirement in readme

CMakeLists.txt

@@ -7,6 +7,7 @@ project(tdlib-purple VERSION ${VERSION} LANGUAGES CXX)
 find_package(Td REQUIRED)
 math(EXPR TDLIB_VERSION_NUMBER "10000*${Td_VERSION_MAJOR} + 100*${Td_VERSION_MINOR} + ${Td_VERSION_PATCH}")
 if (${TDLIB_VERSION_NUMBER} LESS 10709)
+    # Error message must begin with "tdlib version" for a grep command from readme
     message(FATAL_ERROR "tdlib version 1.7.9 or newer is required")
 endif (${TDLIB_VERSION_NUMBER} LESS 10709)
 

README.md

@@ -42,7 +42,12 @@ The debug log contains a lot of private information such as names and phone numb
 
 ## Building
 
-TDLib should be prebuilt and installed somewhere (requires C++14):
+Compatible version of TDLib should be prebuilt and installed somewhere (requires C++14). Version requirement can be found in CMakeLists.txt:
+```
+grep -o "tdlib version.*" CMakeLists.txt
+```
+
+TDLib can be built like this:
 ```
 cd <path to TDLib sources>
 mkdir build
@@ -51,7 +56,8 @@ cmake -DCMAKE_BUILD_TYPE=Release ..
 make
 make install DESTDIR=/path/to/tdlib
 ```
-Also see [building](https://github.com/tdlib/td#building) for additional details on TDLib building.
+
+See also [building](https://github.com/tdlib/td#building).
 
 libtgvoip is required for voice calls.