Add compatibility section to readme

To bump MSRV is considered a breaking change.

Author: taiki-e

README.md

@@ -96,6 +96,10 @@ Next, add this to your crate:
 extern crate crossbeam;
 ```
 
+## Compatibility
+
+The minimum supported Rust version is 1.28. Any change to this is considered a breaking change.
+
 ## Contributing
 
 Crossbeam welcomes contribution from everyone in the form of suggestions, bug reports,

crossbeam-channel/README.md

@@ -51,6 +51,10 @@ Next, add this to your crate:
 extern crate crossbeam_channel;
 ```
 
+## Compatibility
+
+The minimum supported Rust version is 1.28. Any change to this is considered a breaking change.
+
 ## License
 
 Licensed under either of

crossbeam-deque/README.md

@@ -30,6 +30,10 @@ Next, add this to your crate:
 extern crate crossbeam_deque;
 ```
 
+## Compatibility
+
+The minimum supported Rust version is 1.28. Any change to this is considered a breaking change.
+
 ## License
 
 Licensed under either of

crossbeam-epoch/README.md

@@ -37,6 +37,10 @@ Next, add this to your crate:
 extern crate crossbeam_epoch as epoch;
 ```
 
+## Compatibility
+
+The minimum supported Rust version is 1.28. Any change to this is considered a breaking change.
+
 ## License
 
 Licensed under either of

crossbeam-queue/README.md

@@ -37,7 +37,7 @@ extern crate crossbeam_queue;
 
 ## Compatibility
 
-The minimum supported Rust version is 1.28.
+The minimum supported Rust version is 1.28. Any change to this is considered a breaking change.
 
 ## License
 

crossbeam-skiplist/README.md

@@ -33,6 +33,10 @@ extern crate crossbeam_skiplist;
 ```
 -->
 
+## Compatibility
+
+The minimum supported Rust version is 1.28. Any change to this is considered a breaking change.
+
 ## License
 
 Licensed under either of

crossbeam-utils/README.md

@@ -57,6 +57,10 @@ Next, add this to your crate:
 extern crate crossbeam_utils;
 ```
 
+## Compatibility
+
+The minimum supported Rust version is 1.28. Any change to this is considered a breaking change.
+
 ## License
 
 Licensed under either of