docs: add release notes for versions 1.7.3, 1.7.4, and 1.7.5

They were forgotten when those releases were made.

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>

README.md

@@ -31,6 +31,26 @@ For patch submission instructions, see:
 
   https://www.x.org/wiki/Development/Documentation/SubmittingPatches
 
+## Release 1.7.5
+
+ * Avoids a segfault when an invalid name is used for opening a display.
+
+## Release 1.7.4
+
+ * Fixes the "Unknown sequence number" error by allowing backward jumps
+   in the sequence number when widening it.
+ * Any changes to virtual modifiers get propagated properly.
+ * Greek case-conversion tables were updated to Unicode Data 14.0.
+ * Compose sequences for  ☮  🄯  ⇐  ⇑  ⇓  were added,
+   being the following: OY, ()), =<, =^, and =v.
+ * Hammer-and-sickle can be composed with question mark plus backslash.
+
+## Release 1.7.3
+
+ * Fixes a hanging issue in _XReply() where the replying thread would
+   wait for an event when another thread was already waiting for one.
+ * Avoids a crash when the X connection gets broken while closing down.
+
 ## Release 1.7.2
 
 This is a bug fix release, correcting a regression introduced by and