Updated README.md and .gitignore files

-Fixed issue where `gdpm` would to work because of missing directory
-Changed `constexpr const char *` constants to use `const std::string`
string instead to use with `HOME` variable
-Changed where packages are stored. Now, they should stored in the
user's `$HOME/.config/gdpm/packages.db` by default. This may change to
use `$HOME/.cache/gdpm/packages.db` by default in the future.

.gitignore

@@ -1,8 +1,9 @@
 build/**
+bin/gdpm
 tests
 */tmp/**
 vgcore.*
 .cache
 .vscode
 config.json
-*.txt
+*.txt