README: document the package initialization order caveat

Keeping the original lexical sorting of Go packages would be very hard,
as a Go program may import an unknown number of Go packages,
and we load and obfuscate one package at a time by design.

One option would be to load all packages upfront when obfuscating
main packages, but that would break the per-package caching of
ofbuscated Go packages, causing a huge slow-down in builds.

Another option would be to not obfuscate import paths,
which would clearly cause a worsening of the obfuscation quality.

The third option is to not attempt to keep the original order,
and document that as a caveat in the README.
I suspect the vast majority of Go projects won't be affected by this,
and those few that might be can always use imports to enforce the order.

Closes burrowers#693, per the decision above to not change what we do.

README.md

@@ -172,6 +172,11 @@ to document the current shortcomings of this tool.
    var _ = reflect.TypeOf(Message{})
    ```
 
+* Go programs [are initialized](https://go.dev/ref/spec#Program_initialization) one package at a time,
+  where imported packages are always initialized before their importers,
+  and otherwise they are initialized in the lexical order of their import paths.
+  Since garble obfuscates import paths, this lexical order may change arbitrarily.
+
 * Go plugins are not currently supported; see [#87](https://github.com/burrowers/garble/issues/87).
 * Garble requires `git` to patch the linker. That can be avoided once go-gitdiff supports [non-strict patches](https://github.com/bluekeyes/go-gitdiff/issues/30).