Add callouts explaining branches

Addresses #54.

Author: alex-ball

episodes/02-getting-started.md

@@ -167,9 +167,22 @@ No commits yet
 nothing to commit (create/copy files and use "git add" to track)
 ```
 
-The output tells us that we are on the main branch (more on this later) and that we have nothing to commit (no
+The output tells us that we are on the main branch and that we have nothing to commit (no
 unsaved changes).
 
+:::::::::::::::::::::::::::::::::::::::::  callout
+
+## Branches and commits
+
+Commits in Git are like snapshots of our project.
+They are ordered into sequences, or "branches", with each commit pointing back to the one before it in the branch.
+Git keeps track of the last commit in each branch with a special label;
+in this lesson we mostly use one branch with the label "main".
+
+![](fig/git-branch.svg){alt='The label ‘main’ points to commit c2; c2 points to its parent c1, which points to c0.'}
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
 ### Two steps: Adding and committing
 
 We will now create and save our first project file. This is a two-step process. First, we **add** any files for which

episodes/05-github-pages.md

@@ -133,6 +133,20 @@ it's more practical to grant everyone access to commit directly instead.
 
 ::::::::::::::::::::::::::::::::::::::::::::::::::
 
+:::::::::::::::::::::::::::::::::::::::::  callout
+
+## Branching and merging
+
+Branches get their name because you can create a new one (say, "patch")
+that splits off from an existing one like "main",
+allowing different sets of changes to be made in parallel.
+Later on, Git can take the changes from "patch" and merge (copy) them into the "main" branch.
+In the last challenge, GitHub did all this for us in the background.
+
+![](fig/git-branches.svg){alt='The label ‘main’ points to commit c5; c5 points to its parent c3, which points to c1, which points to c0. The label ‘patch’ points to commit c4, which points to c2, which points to c1. Commit c5 also points to c4 as a secondary parent, indicating ‘patch’ was merged into ‘main’.'}
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
 :::::::::::::::::::::::::::::::::::::::: challenge
 
 ## Optional challenge: Contributing to a page owned by someone else (slightly more complicated way)