docs(contrib): add Minimum Support Rust Version (MSRV) policy

.github/workflows/CI.yml

@@ -52,7 +52,7 @@ jobs:
           - stable
           - beta
           - nightly
-          - 1.46
+          - 1.46 # keep in sync with MSRV.md dev doc
 
         os:
           - ubuntu-latest

dev/MSRV.md

@@ -0,0 +1,9 @@
+# Minimum Support Rust Version (MSRV)
+
+hyper's current policy is to always support a Rust version at least 6 months
+old. That is, a compiler version released within the last 6 months can compile
+hyper. It is possible that an older compiler can work, but that is not
+guaranteed. We try to increase the MSRV responsibly, only when a significant
+new feature is needed.
+
+The current MSRV is: **1.46**.