docs(3p/lisp/mime4cl): describe changes compared to original version r/6182

Spell out that “may diverge” is more of a “has diverged by now”. We are
essentially maintaining a fork of mime4cl.

Change-Id: I9049e8296a666c3d1b08eae28813147f360771ef
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8621
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

2 files changed, 27 insertions, 7 deletions

diff --git a/third_party/lisp/mime4cl/README b/third_party/lisp/mime4cl/README
deleted file mode 100644
index 73f0efbda9d5..000000000000
--- a/third_party/lisp/mime4cl/README
+++ /dev/null
@@ -1,7 +0,0 @@
-MIME4CL is a Common Lisp library for dealing with MIME messages.
-It has originally been written by Walter C. Pelissero and vendored
-into depot as upstream has become inactive and provides no repo
-of any kind. Upstream and depot version may diverge.
-
-Upstream Website: http://wcp.sdf-eu.org/software/#mime4cl
-Vendored Tarball: http://wcp.sdf-eu.org/software/mime4cl-20150207T211851.tbz
diff --git a/third_party/lisp/mime4cl/README.md b/third_party/lisp/mime4cl/README.md
new file mode 100644
index 000000000000..2704d481ed3c
--- /dev/null
+++ b/third_party/lisp/mime4cl/README.md
@@ -0,0 +1,27 @@
+# mime4cl
+
+`MIME4CL` is a Common Lisp library for dealing with MIME messages. It was
+originally been written by Walter C. Pelissero and vendored into depot
+([mime4cl-20150207T211851.tbz](http://wcp.sdf-eu.org/software/mime4cl-20150207T211851.tbz)
+to be exact) as upstream has become inactive. Its [original
+website](http://wcp.sdf-eu.org/software/#mime4cl) can still be accessed.
+
+The depot version has since diverged from upstream. Main aims were to improve
+performance and reduce code size by relying on third party libraries like
+flexi-streams. It is planned to improve encoding handling in the long term.
+Currently, the library is being worked on intermittently and not very well
+tested—**it may not work as expected**.
+
+## Differences from the original version
+
+* `//nix/buildLisp` is used as the build system. ASDF is currently untested and
+  may be broken.
+
+* The dependency on [sclf](http://wcp.sdf-eu.org/software/#sclf) has been
+  eliminated by inlining the relevant parts.
+
+* `MY-STRING-INPUT-STREAM`, `DELIMITED-INPUT-STREAM`,
+  `CHARACTER-INPUT-ADAPTER-STREAM`, `BINARY-INPUT-ADAPTER-STREAM` etc. have been
+  replaced by (thin wrappers around) flexi-streams. In addition to improved
+  handling of encodings, this allows using `READ-SEQUENCE` via the gray stream
+  interface.