List all custom assertions in submodule readme

This allows you to click on the sub-package on code.dlang.org and get a
list. This is something have now done a few times (instinctively, the
affordance is definitely there for it) but I never found anything and
had to go source diving for the functions.

Extracted with the following commands from the git root dir:
```
PATTERN='^\s*(?:[A-Za-z_@()\s]*\s)?(?:auto|void)\s+(should[A-Z][A-Za-z_0-9]*)'
FILE=subpackages/assertions/source/unit_threaded/assertions.d
rg -P --no-line-number --only-matching --replace '$1' "$PATTERN" "$FILE" \
	| awk '!seen[$0]++' \
	| sed 's/^/- /'
```

Author: Inkrementator

subpackages/assertions/README.md

@@ -0,0 +1,32 @@
+This module contains the following assertions:
+
+- shouldBeTrue
+- shouldBeFalse
+- shouldEqual
+- shouldNotEqual
+- shouldBeNull
+- shouldNotBeNull
+- shouldBeIn
+- shouldNotBeIn
+- shouldThrow
+- shouldThrowExactly
+- shouldNotThrow
+- shouldThrowWithMessage
+- shouldApproxEqual
+- shouldBeEmpty
+- shouldNotBeEmpty
+- shouldBeGreaterThan
+- shouldBeSmallerThan
+- shouldBeSameSetAs
+- shouldNotBeSameSetAs
+- shouldBeSameJsonAs
+- shouldBeBetween
+- shouldApprox
+
+It also contains the `should` -testing DSL. I.e.:
+```
+1.should == 1;
+1.should.not == 2;
+1.should in [1, 2, 3];
+4.should.not in [1, 2, 3];
+```