[LLD] Add release notes for MinGW for the 10.x branch

[mstorsjo]

Summary

I've tried to fill in commit references like has been done in earlier lld release notes (contrary to most of the other projects' release notes).

[mstorsjo]

Committed in https://reviews.llvm.org/rG424babb89ad2.

lld/docs/ReleaseNotes.rst

 COFF Improvements
 -----------------
 
 * ...
 
 MinGW Improvements
 ------------------
 
-* ...
+* Allow using custom .edata sections from input object files (for use
+  by Wine)
+  (`dadc6f248868 <https://reviews.llvm.org/rGdadc6f248868>`)
+
+* Don't implicitly create import libraries unless requested
+  (`6540e55067e3 <https://reviews.llvm.org/rG6540e55067e3>`)
+
+* Support merging multiple resource object files
+  (`3d3a9b3b413d <https://reviews.llvm.org/rG3d3a9b3b413d>`)
+  and properly handle the default manifest object files that GCC can pass
+  (`d581dd501381 <https://reviews.llvm.org/rGd581dd501381>`)
+
+* Demangle itanium symbol names in warnings/error messages
+  (`a66fc1c99f3e <https://reviews.llvm.org/rGa66fc1c99f3e>`)
+
+* Print source locations for undefined references and duplicate symbols,
+  if possible
+  (`1d06d48bb346 <https://reviews.llvm.org/rG1d06d48bb346>`)
+  and
+  (`b38f577c015c <https://reviews.llvm.org/rGb38f577c015c>`)
+
+* Look for more filename patterns when resolving ``-l`` options
+  (`0226c35262df <https://reviews.llvm.org/rG0226c35262df>`)
+
+* Don't error out on duplicate absolute symbols with the same value
+  (which can happen for the default-null symbol for weak symbols)
+  (`1737cc750c46 <https://reviews.llvm.org/rG1737cc750c46>`)
 
 MachO Improvements
 ------------------
 
 * Item 1.
 
 WebAssembly Improvements
 ------------------------
 
 * `__data_end` and `__heap_base` are no longer exported by default,
   as it's best to keep them internal when possible. They can be
   explicitly exported with `--export=__data_end` and
   `--export=__heap_base`, respectively.