update: README.md

precise usage instruction and add similar projects

Author: luckasRanarison

README.md

@@ -78,6 +78,10 @@ Here is the default configuration:
 }
 ```
 
+## Usage
+
+To use the documentations from nvim-devdocs you have to **install** the documentation using `:DevdocsInstall`.
+
 ## Commands
 
 Available commands:
@@ -108,6 +112,10 @@ The HTML converter is still experimental, and not all documentation has been tho
 
 Pull requests and feature requests are welcome!
 
+## Similar projects
+
+- [nvim-telescope-zeal-cli](https://gitlab.com/ivan-cukic/nvim-telescope-zeal-cli) Show Zeal documentation pages in Neovim Telescope.
+
 ## Credits
 
 - [The DevDocs project](https://github.com/freeCodeCamp/devdocs) for the documentations.