chore(readme): document methods

README.md

@@ -28,6 +28,33 @@ gpg-agent:         353 hashes (       11.7519/s)
 ```
 <small>The asterisked sequoia is using the `crypto-rust` feature.</small>
 
+## Methods
+
+`git-vain` can use one of several different methods to generate different hashes
+for a commit.
+
+**Note that none of these are necessary when using PGP signing.** Since the
+signature will be different every time, the hash will also be different every
+time. If signing is enabled, none of these methods will be used.
+
+### `increment` and `decrement`
+Increases or decreases the commit's timestamp by one second each try. This is
+not recommended for longer prefixes (or really at all, in the author's opinion).
+
+### `counter`
+Appends an increasing counter to the end of the commit message (in the body).
+
+### `random`
+Appends a random 32-character hexadecimal string to the end of the commit
+message (in the body).
+
+### `header`
+Adds an additional `xvain` header to the commit, which contains an increasing
+counter. Git ignores additional headers that aren't `gpgsig`, so this does work
+and is considered valid by `git fsck`. In the author's opinion, it is doubtful
+that Git would change the way headers are parsed, but it is possible, and that
+would break this method (and possibly the commits created by it).
+
 ## PGP performance
 
 Signing commits drastically reduces the speed at which `git-vain` operates. By