Add CHANGELOG entry and comments

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+- On X11, input method creation first tries to use the value from the user's `XMODIFIERS` environment variable, so application developers should no longer need to manually call `XSetLocaleModifiers`. If that fails, fallbacks are tried, which should prevent input method initialization from ever outright failing.
+
 # Version 0.11.3 (2018-03-28)
 
 - Added `set_min_dimensions` and `set_max_dimensions` methods to `Window`, and implemented on Windows, X11, Wayland, and OSX.

src/platform/linux/x11/mod.rs

@@ -1012,6 +1012,10 @@ impl Window {
 
             let mut im: ffi::XIM = ptr::null_mut();
 
+            // Setting an empty locale results in the user's XMODIFIERS environment variable being
+            // read, which should result in the user's configured input method (ibus, fcitx, etc.)
+            // being used. If that fails, we fall back to internal input methods which should
+            // always be available.
             for modifiers in &[b"\0" as &[u8], b"@im=local\0", b"@im=\0"] {
                 if !im.is_null() {
                     break;
@@ -1027,6 +1031,10 @@ impl Window {
             }
 
             if im.is_null() {
+                // While it's possible to make IME optional and handle this more gracefully, it's
+                // not clear in what situations the fallbacks wouldn't work. Therefore, this panic
+                // is left here so that if it ever fails, someone will hopefully tell us about it,
+                // and we'd have a better grasp of what's necessary.
                 panic!("XOpenIM failed");
             }