Include linear operator in installation instructions (#1385)

Summary:
Some of the latest features may rely on bleeding edge features in linear operator.

Pull Request resolved: #1385

Reviewed By: Balandat, SebastianAment

Differential Revision: D39368817

Pulled By: saitcakmak

fbshipit-source-id: 6a0dfaf4b3d07a3d09d4aeb2d581b02f9505853f

Author: saitcakmak

README.md

@@ -94,6 +94,7 @@ running into the occasional bug here or there), you can install the latest
 development version directly from GitHub (this will also require installing
 the current GPyTorch development version):
 ```bash
+pip install --upgrade git+https://github.com/cornellius-gp/linear_operator.git
 pip install --upgrade git+https://github.com/cornellius-gp/gpytorch.git
 pip install --upgrade git+https://github.com/pytorch/botorch.git
 ```