Updating histoby-substring-search README to instruct user to load plugins via .zshrc. Closes #1832.

plugins/history-substring-search/README

-To activate this script, load it into an interactive ZSH session:
-
-  % source history-substring-search.zsh
-
-See the "history-substring-search.zsh" file for more information:
-
-  % sed -n '2,/^$/s/^#//p' history-substring-search.zsh | more

plugins/history-substring-search/README.markdown

+To activate this script, please include it the `plugins` variable within `~/.zshrc`
+
+  `plugins=(git history-substring-search.zsh)`
+
+See the "history-substring-search.zsh" file for more information:
+
+  `sed -n '2,/^$/s/^#//p' history-substring-search.zsh | more`