HsShellScript is cabalized, which means that building and installing is a matter of some calls to cabal. However, it also provides a Makefile, which further simplifies the installation.
In order to install, unpack the source distribution somewhere. Go to the directory, and call make. This will compile the library. If you want to install it locally, in your user account, then do make install-user. This calls cabal install --user. If you want to install the library globally, do make install-global. You will be prompted for the root password, which is needed in this case. This calls sudo cabal install --global.
If installed as a user package, then the location of the API documentation is ~/.cabal/share/doc/hsshellscript-3.3.1/html/index.html. If it is installed globaly, it is /usr/local/share/doc/hsshellscript-3.3.1/html/index.html.
Cabal's Simple Build Infrastructure doesn't provide any means to add extra documenation to a project. Therefore the user manual isn't installed by default. If you need it, you can just copy the "manual" directory anywhere you like, but the Makefile privides the make target install-manual, which copies the files to /usr/local/share/hsshellscript/manual.
Further information about the Cabal can be found here: The Haskell Cabal