add documentation about extensions

Signed-off-by: Gordon Hecker <gh@emlix.com>

diff --git a/README.extensions b/README.extensions
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+e2factory has extension support now.
+
+e2factory extensions are simply additional code that is installed in 
+addition to the e2factory core.
+
+extensions can provide a local and a global part. The requirements are
+
+ * a git repository, located next to the e2factory core one, if the 
+   extension should be automatically installed with the local tools
+ * the extensions' Makefile must provide the targets local and install-local
+   to do local install (via e2-install-e2)
+ * the extensions' Makefile should provide the targets all and install to
+   install globally.
+ * Either part may be empty (i.e. build and install nothing). In other words:
+   Extensions may provide code for local install or code for global install
+   or both.
+ * extensions are configured in .e2/extensions, this config file replaces
+   .e2/e2version in versions of e2factory with extension support.
+ * It is planned to provide a plugin interface