More detailed document for how to install libgit2 on unix/linux system #41

Many developer ran into problem during libgit2 installation.

1. The existing libgit2 installed by package manager is most likely not of version 1.3.0, one could use pkg-config --modversion libgit2 to make sure of that
2. When existing version doesn't fit our requirement, provide a guide to uninstall existing version, and adopt the Manual Compilation approach.
3. Prompt developer to setup PKG_CONFIG_PATH and LD_LIBRARY_PATH environment variable for Manual Compilation approach.
4. Provide some common error message screen shot for developer to check if they are having the same kind of problem