Add .plugin file and installation instructions for Oh My Zsh (#104).

Makefile

@@ -17,9 +17,11 @@ HEADER_FILES := \
 	LICENSE
 
 PLUGIN_TARGET := zsh-autosuggestions.zsh
+OH_MY_ZSH_LINK_TARGET := zsh-autosuggestions.plugin.zsh
 
 ALL_TARGETS := \
-	$(PLUGIN_TARGET)
+	$(PLUGIN_TARGET) \
+	$(OH_MY_ZSH_LINK_TARGET)
 
 all: $(ALL_TARGETS)
 
@@ -27,6 +29,9 @@ $(PLUGIN_TARGET): $(HEADER_FILES) $(SRC_FILES)
 	cat $(HEADER_FILES) | sed -e 's/^/# /g' >> $@
 	cat $(SRC_FILES) >> $@
 
+$(OH_MY_ZSH_LINK_TARGET): $(PLUGIN_TARGET)
+	ln -s $(PLUGIN_TARGET) $@
+
 .PHONY: clean
 clean:
 	rm $(ALL_TARGETS)