Add libs folder as a placeholder and a README file

Author: mxamin

.gitignore

@@ -14,3 +14,7 @@
 *.pyo
 *.egg*
 *.so
+
+# Downloaded dependency archives
+/libs/*
+!/libs/README.md

libs/README.md

@@ -0,0 +1,50 @@
+# Dependency archive cache
+
+This directory is used as the local cache for third-party libraries needed by
+the static build tooling.
+
+## How it works
+
+`build_libs_xmlsec.py` and `LibXmlsecDependencyBuilder` look in `libs/` first.
+If a matching archive is already present here, that file is reused. If not, the
+build tooling downloads the archive into this directory and then continues.
+
+The default lookup path is:
+
+```bash
+libs/
+```
+
+You can override it with either:
+
+```bash
+python build_libs_xmlsec.py --libs-dir /path/to/cache
+```
+
+or:
+
+```bash
+export PYXMLSEC_LIBS_DIR=/path/to/cache
+```
+
+## What belongs here
+
+Store downloaded source or binary archives here, for example:
+
+- `openssl*.tar.gz`
+- `zlib*.tar.gz`
+- `libiconv*.tar.gz`
+- `libxml2*.tar.xz`
+- `libxslt*.tar.xz`
+- `xmlsec1*.tar.gz`
+- Windows binary archives such as `libxml2-<version>.<suffix>.zip`
+
+Extracted build artifacts do not belong in this directory. Those are created
+under `build/tmp/libs/`.
+
+## Why keep this directory
+
+- Speeds up local rebuilds by reusing previously downloaded archives.
+- Matches the CI cache strategy in `.github/workflows/cache_libs.yml`.
+- Makes it possible to pre-populate dependency archives for offline or
+  repeatable builds.