Elaborate instructions on which files to #include for which functions/operators

[607011]

The README states that you only have to #include a single header file. That's not true. You also have to include one or more header files from the directories below the one with BigInt.hpp depending one the needs of your code.

[oviano]

The single header file is not included in the Git repo. You either need to download that from the Releases, or generate it yourself by typing 'make release" on either macOS or Linux.

[faheel]

@607011 The README mentions that you'll need to download the single-include header file (also includes the link for the same) and include that. No need to include any other file.

I'll update it to make it clearer that the file needs to be downloaded from GitHub releases. Currently only the link points to GitHub releases, there's no textual reference.