bootstrap.sh: add comments on SCOPE_OF_INSTALLATION for custom package

database install

cabal-install/bootstrap.sh

@@ -28,6 +28,20 @@ CURL="${CURL:-curl}"
 FETCH="${FETCH:-fetch}"
 TAR="${TAR:-tar}"
 GZIP_PROGRAM="${GZIP_PROGRAM:-gzip}"
+
+# SCOPE_OF_INSTALLATION can be set on the command line to use/install
+# the libaries needed to build cabal-install to a custom package
+# database instead of the user or global package database
+# e.g.
+# ghc-pkg init /my/package/database
+# SCOPE_OF_INSTALLATION=/my/package/database ./bootstrap.sh
+#
+# you can use this with PREFIX also, e.g.
+# ghc-pkg init /my/prefix/packages.conf.d
+# PREFIX=/my/prefix SCOPE_OF_INSTALLATION=/my/prefix/packages.conf.d ./bootstrap.sh
+#
+# if you use the --global or --user arguments, this will override the
+# SCOPE_OF_INSTALLATION and not use a custom package database
 SCOPE_OF_INSTALLATION="${SCOPE_OF_INSTALLATION:---user}"
 DEFAULT_PREFIX="${HOME}/.cabal"