Update README.md

Add Manisha (monitoring tool)

README.md

@@ -148,6 +148,7 @@ Repo mirrors:
 - [md2gmn](https://git.tdem.in/tdemin/gmnhg) (Go) - renders Markdown text to Gemini pages.
 - [spacewalk](https://tildegit.org/sloum/spacewalk) (Go) - Page-hash based feed aggregator for gemini.
 - [CAPCOM](https://tildegit.org/solderpunk/CAPCOM) (Python) - Atom feed aggregator for gemini.
+- [Manisha](https://framagit.org/bortzmeyer/manisha) - A Nagios (and compatible, such as Icinga) plugin to monitor Gemini servers
 
 ### Syntax Highlighting
 - [gemini-vim-syntax](https://tildegit.org/sloum/gemini-vim-syntax) - text/gemini syntax highlighting for vim.