conf: document acquire_by_hash_algorithms in the default aptly.conf

The key added in #1626 was missing from debian/aptly.conf, which aptly
writes out as ~/.aptly.conf on first run, so new installs never saw it.

debian/aptly.conf

@@ -32,6 +32,17 @@ database_open_attempts: -1
 architectures:
 #    - amd64
 
+# Hash algorithms to write Acquire-By-Hash entries for, when a publish
+# has Acquire-By-Hash enabled. Each algorithm costs a full set of index
+# copies on every publish, which is felt most on S3-like storage where
+# every write is a round trip. An empty list defaults to all four.
+# Modern apt uses SHA256; MD5Sum and SHA1 are deprecated.
+acquire_by_hash_algorithms:
+    - MD5Sum
+    - SHA1
+    - SHA256
+    - SHA512
+
 # OBSOLETE
 # in aptly up to version 1.0.0, package files were stored in internal package pool
 # with MD5-dervied path, since 1.1.0 package pool layout was changed;

system/t02_config/CreateConfigTest_gold

@@ -32,6 +32,17 @@ database_open_attempts: -1
 architectures:
 #    - amd64
 
+# Hash algorithms to write Acquire-By-Hash entries for, when a publish
+# has Acquire-By-Hash enabled. Each algorithm costs a full set of index
+# copies on every publish, which is felt most on S3-like storage where
+# every write is a round trip. An empty list defaults to all four.
+# Modern apt uses SHA256; MD5Sum and SHA1 are deprecated.
+acquire_by_hash_algorithms:
+    - MD5Sum
+    - SHA1
+    - SHA256
+    - SHA512
+
 # OBSOLETE
 # in aptly up to version 1.0.0, package files were stored in internal package pool
 # with MD5-dervied path, since 1.1.0 package pool layout was changed;