Add MSRV section to readme

Author: stevenroose

README.md

@@ -34,6 +34,24 @@ coins in a given Bitcoin transaction
 More information can be found in [the documentation](https://docs.rs/miniscript)
 or in [the `examples/` directory](https://github.com/apoelstra/rust-miniscript/tree/master/examples)
 
+
+## Minimum Supported Rust Version (MSRV)
+This library should always compile with any combination of features on **Rust 1.29**.
+
+Because some dependencies have broken the build in minor/patch releases, to
+compile with 1.29.0 you will need to run the following version-pinning command:
+```
+cargo update -p cc --precise "1.0.41" --verbose
+```
+
+In order to use the `use-serde` feature or to build the unit tests with 1.29.0,
+the following version-pinning commands are also needed:
+```
+cargo update --package "serde" --precise "1.0.98"
+cargo update --package "serde_derive" --precise "1.0.98"
+```
+
+
 ## Contributing
 Contributions are generally welcome. If you intend to make larger changes please
 discuss them in an issue before PRing them to avoid duplicate work and