docs: add nix instructions

Author: tennox

README.md

@@ -4,14 +4,15 @@
 ###### explainshell.com CLI implementation in Rust
 _____________
 
-Needs the Rust Toolchain (available at [https://rustup.rs/]() ).
 
-##### Installation:  
-```cargo install --git https://github.com/p1g30n/explainshell-cli```  
+##### Try it out:  
+```nix shell github:thepigeonoftime/explainshell-cli```  
 
->note that ~/.cargo/bin/ must be in your $PATH
+##### Installation:  
+Use your favorite way to install nix packages, or simply:
+```nix profile install github:thepigeonoftime/explainshell-cli```  
 
-##### Usage:  
+#### Usage:  
 ```explain COMMAND [ARGS]``` (combining arguments works)
 
 ##### Example:  
@@ -37,4 +38,4 @@ __________________________________________________
 -f, --file ARCHIVE
       use archive file or device ARCHIVE
 __________________________________________________
-```
+```