fish: add info for universal variables

applications/fish

@@ -9,10 +9,16 @@ if status is-interactive
 end
 
 # bootstrapping information
+# required universal variables:
+# - _agw_dir_bin     = volatile binary storage
+# - _agw_dir_rc      = the directory holding this repository
+# - _agw_dir_scripts = the directory for user-facing scripts
+# - _agw_dir_src     = non-volatile source code storage
 switch (uname)
 	case Linux
 		set -gx CC clang
 		set -gx CXX clang++
+		set -gx GOPATH ~/go
 		set -Ux _agw_dir_bin ~/bin
 		set -Ux _agw_dir_rc ~/src/git-scripts
 		set -Ux _agw_dir_scripts ~/src/scripts