[pgadmin] patch: supported cmake version

While updating the Debian package I noticed that pgadmin requires at
least cmake 3.3:

CMake Error at cmake/FindBoost.cmake:238 (cmake_policy):
Policy "CMP0057" is not known to this version of CMake.

https://cmake.org/cmake/help/v3.3/policy/CMP0057.html

CMake 3.3 adds support for the new IN_LIST operator.

This should be noted in the documentation:

diff --git a/README b/README
index 610f4ba..6c583b4 100644
--- a/README
+++ b/README
@@ -15,7 +15,7 @@ Building pgAgent
You will need:

- A C/C++ compiler, such as GCC or Microsoft Visual C++ on Windows.
-- CMake 2.6 (from www.cmake.org)
+- CMake 3.3 or higher (from www.cmake.org)
- A Boost library 1.41 or higher installation
- A PostgreSQL 8.3 or higher installation

Christoph