docs: Add conda-forge install to instructions (#235)

* Add install instructions from conda-forge for using 'pixi global' to the README.
  This has the advantage of installing in user space in an isolated environment
  that can't be broken by other package changes.
   - c.f. https://github.com/conda-forge/brev-feedstock

Author: matthewfeickert

README.md

@@ -10,6 +10,14 @@
 
 ## Install the cli
 
+### From conda-forge
+
+To globally install `brev` [from conda-forge](https://github.com/conda-forge/brev-feedstock/) in an isolated environment with [`Pixi`](https://pixi.sh/), run
+
+```
+pixi global install brev
+```
+
 ### MacOS and Linux
 
 ```