Merge pull request #118 from Kixunil/document-source-date-epoch

Document `SOURCE_DATE_EPOCH`

README.md

@@ -127,6 +127,7 @@ fn main() {
     println!("{}", build::CARGO_TREE);          // e.g. the output of '$ cargo tree'
 
     println!("{}", build::PROJECT_NAME);        // your project name, e.g. 'shadow-rs'
+    // Time respects SOURCE_DATE_EPOCH environment variable - see below
     println!("{}", build::BUILD_TIME);          // time when start build occurred, e.g. '2020-08-16 14:50:25'
     println!("{}", build::BUILD_TIME_2822);     // time when start build occurred by rfc2822, e.g. 'Thu, 24 Jun 2021 21:33:59 +0800'
     println!("{}", build::BUILD_TIME_3339);     // time when start build occurred by rfc3339, e.g. '2021-06-24T21:33:59.972494+08:00'
@@ -136,6 +137,11 @@ fn main() {
 }
 ```
 
+#### Reproducibility
+
+This tool includes the current time in the binary which would normally make it non-reproducible.
+However, it respects the [`SOURCE_DATE_EPOCH` variable](https://reproducible-builds.org/docs/source-date-epoch/) - if set to a Unix timestamp it will override the value of build time.
+
 ## Clap Example
 
 You also can use shadow-rs with [`clap`](https://github.com/baoyachi/shadow-rs/blob/master/example_shadow/src/main.rs).