[libxml2] Add note about optimization flags

[Nick Wellnhofer <nwellnhof src gnome org>]

commit b26d581d66bd5258307761c421e22c17989c838c
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sun Feb 20 21:26:50 2022 +0100

    Add note about optimization flags

 README.md | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/README.md b/README.md
index ce83d620..ad423e68 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,11 @@ configure and build the library:
     ./configure [possible options]
     make
 
+Note that by default, no optimization options are used. You have to
+enable them manually, for example with:
+
+    CFLAGS='-O2 -fno-semantic-interposition' ./configure
+
 Now you can run the test suite with:
 
     make check