docs: add supported rust versions (#185)

* docs: add supported rust versions

* Update README.md

Author: prasannavl

README.md

@@ -84,6 +84,19 @@ Read about the design here:
 - [Middleware in Tide](https://rustasync.github.io/team/2018/11/07/tide-middleware.html)
 - [Tide's evolving middleware approach](https://rustasync.github.io/team/2018/11/27/tide-middleware-evolution.html)
 
+### Supported Rust Versions
+
+Tide is built against the latest Rust nightly releases and as such, due to it's use of `std` futures,
+it has the following specific breakpoints that align with std future API changes:
+
+Tide | Rust
+--- | ---
+≤ v0.1.0 | ≤ nightly-2019-04-07
+≥ v0.1.1 | ≥ nightly-2019-04-08
+
+
+_**Note:** Since these are changes are in `std`, projects with dependencies that use conflicting versions of `std::futures` will not build successfully._
+
 ## Contributing
 Want to join us? Check out our [The "Contributing" section of the
 guide][contributing] and take a look at some of these issues: