🏗 Add xdelta3 dependency

lib/xdelta3/cpp-btree/README

@@ -0,0 +1,31 @@
+This library is a C++ template library and, as such, there is no
+library to build and install.  Copy the .h files and use them!
+
+See http://code.google.com/p/cpp-btree/wiki/UsageInstructions for
+details.
+
+----
+
+To build and run the provided tests, however, you will need to install
+CMake, the Google C++ Test framework, and the Google flags package.
+
+Download and install CMake from http://www.cmake.org
+
+Download and build the GoogleTest framework from
+http://code.google.com/p/googletest
+
+Download and install gflags from https://code.google.com/p/gflags
+
+Set GTEST_ROOT to the directory where GTEST was built.
+Set GFLAGS_ROOT to the directory prefix where GFLAGS is installed.
+
+export GTEST_ROOT=/path/for/gtest-x.y
+export GFLAGS_ROOT=/opt
+
+cmake . -Dbuild_tests=ON
+
+For example, to build on a Unix system with the clang++ compiler,
+
+export GTEST_ROOT=$(HOME)/src/googletest
+export GFLAGS_ROOT=/opt
+cmake . -G "Unix Makefiles" -Dbuild_tests=ON -DCMAKE_CXX_COMPILER=clang++