Add to contributing and cabal-testsuite's readme

CONTRIBUTING.md

@@ -395,9 +395,12 @@ description: {
 }
 ```
 
-Changelogs may also be written in "markdown-frontmatter" format. This is useful if your
-description contains braces, which must be escaped with backslashes in `.cabal` file
-format. The front matter is in YAML syntax, not `.cabal` file syntax, and the file
+Changelogs may also be written in "markdown-frontmatter" format. This is useful
+if your description contains braces, which must be escaped with backslashes in
+`.cabal` file format. Another benefit of using an `.md` extension with your
+changelog is that it will be checked for typos.
+
+The front matter is in YAML syntax, not `.cabal` file syntax, and the file
 _must_ begin with a line containing only hyphens.
 
 ```markdown

cabal-testsuite/README.md

@@ -218,6 +218,10 @@ variants of a command (e.g., `cabal'` rather than `cabal`) and use
 `assertOutputContains`.  Note that this will search over BOTH stdout
 and stderr.
 
+For convenience, paste expected multiline string values verbatim into a text
+file and read these with `readFileVerbatim`. The suggested extension for these
+files are `.expect.txt`.
+
 **How do I skip running a test in some environments?**  Use the
 `skipIf` and `skipUnless` combinators.  Useful parameters to test
 these with include `hasSharedLibraries`, `hasProfiledLibraries`,

changelog.d/pr-10646.md

@@ -42,9 +42,9 @@ expectations for `cabal-testsuite/PackageTests/**/*.test.hs` test scripts might
 be have been easier but for a catch. We run these tests with older `GHC`
 versions so would need to use `-XCPP` for those versions and the C preprocessor
 does not play nicely with string gaps. While it is possible to encode a
-multiline string as a single line with in embedded LF characters or by breaking
-the line up arbitrarily and using `++` concatenation or by calling unlines on a
-list of lines, string gaps are the multiline strings of Haskell prior to
+multiline string as a single line with embedded LF characters or by breaking the
+line up arbitrarily and using `++` concatenation or by calling unlines on a list
+of lines, string gaps are the multiline strings of Haskell prior to
 `-XMultilineStrings`.
 
 To avoid these problems and for the convenience of pasting the expected value