README: rewrite seeds section

A user correctly points out that some sentences were confusing, like:

	It can also make reverse-engineering harder, as an end user could
	guess what version of Go or garble you're using.

Rewrite the whole section. It now also explains what I meant by that
sentence with a practical example.

I also merged the bits about "provide your own seed" and "rotating the
seeds", because they're both talking about the same mechanism.

Fixes #666.

README.md

@@ -103,18 +103,16 @@ which will change if its build input changes: the version of garble, the version
 of Go, the package's source code, or any build parameter such as GOOS or -tags.
 This is a reasonable default since guessing those inputs is very hard.
 
-However, providing your own obfuscation seed via `-seed` brings some advantages.
+You can use the `-seed` flag to provide your own obfuscation randomness seed.
-For example, builds sharing the same seed will produce the same obfuscation,
+Reusing the same seed can help produce the same code obfuscation,
-even if any of the build parameters or versions vary.
+which can help when debugging or reproducing problems.
-It can also make reverse-engineering harder, as an end user could guess what
+Regularly rotating the seed can also help against reverse-engineering in the long run,
-version of Go or garble you're using.
+as otherwise one can look at changes in how Go's standard library is obfuscated
-
+to guess when the Go or garble versions were changed across a series of builds.
-Note that extra care should be taken when using custom seeds.
+
-If a seed used to build a binary gets lost, `garble reverse` will not work.
+To always use a different seed for each build, use `-seed=random`.
-Rotating the seeds can also help against reverse-engineering in the long run,
+Note that extra care should be taken when using custom seeds:
-as otherwise some bits of code may be obfuscated the same way over time.
+if a `-seed` value used in a build is lost, `garble reverse` will not work.
-
-An alternative approach is `-seed=random`, where each build is entirely different.
 
 ### Caveats