Update installation instructions with terminology section

Added common terminology section to installation instructions.

Author: jefftyoder

docs/src/installation_instructions.md

@@ -1,4 +1,4 @@
-# Installation instructions
+## Installation instructions
 
 ClimaAtmos.jl is a [registered Julia package](https://julialang.org/packages/). To install
 
@@ -29,3 +29,12 @@ $ julia --project
 julia> ]
 (ClimaAtmos) pkg> instantiate
 ```
+
+## Some common terminology in ClimaAtmos
+The following terms are frequently used within the source code and between collaborators. Feel free to open a GitHub issue if you come across any other key terms that we've missed here.
+- `Yₜ`: The tendency state vector, where `Yₜ.sfc` components are modified.
+- `Y`: The current state vector.
+- `p`: Cache containing parameters, precomputed fields (radiation fluxes, surface
+       conditions, precipitation fluxes), atmospheric model configurations, and
+       slab model properties.
+- `t`: Current simulation time.