Update appendix-07-nightly-rust.md to add maintenance time section

Update appendix-07-nightly-rust.md
add  maintenance time section

Signed-off-by: Wang Qilin <qilin.wang@huawei.com>

src/appendix-07-nightly-rust.md

@@ -114,6 +114,12 @@ work as expected, you can report it to the team and get it fixed before the
 next stable release happens! Breakage in a beta release is relatively rare, but
 `rustc` is still a piece of software, and bugs do exist.
 
+### Maintenance time
+
+The Rust project only supports the most recent stable version, which means the 
+maintenance time of a stable version is six weeks. When a new stable version 
+released, the old version reaches its EOL(end-of-life). 
+
 ### Unstable Features
 
 There’s one more catch with this release model: unstable features. Rust uses a