Fix the review findings in README.md

The first sentence now names the licence, the kind of program, the
language and the author. The README gains a Design section, and its
sections carry the names REPO_POLICIES.md asks for: Rationale and
Getting started, which now holds the install command. Entrypoints opens
with the Scripts to Rule Them All statement.

Corrected: when a file is read again, the chunk size limits, and the
first prune step, which looks for a snapshot's manifest. Two sentences
that misparsed are reworded.

A test comment that named the README quickstart follows the rename.

Model: fable-5-1

internal/cli/config_test.go

@@ -61,8 +61,8 @@ func TestDefaultConfigTemplateParses(t *testing.T) {
 // testRecipient is a valid age public key.
 const testRecipient = "age1278m9q7dp3chsh2dcy82qk27v047zywyvtxwnj4cvt0z65jw6a7q5dqhfj"
 
-// TestConfigSetRecipientOnFreshConfig follows the README quickstart: on the
-// file `config init` writes, `config set age_recipients.0` and
+// TestConfigSetRecipientOnFreshConfig follows the README's getting started
+// steps: on the file `config init` writes, `config set age_recipients.0` and
 // `config set storage_url` give a config that loads with that recipient.
 // Neither is refused, although the config does not load until both are set;
 // once it loads, a recipient that is not a valid key is.