Small fixes (#1702)

* Add `export` to bash instruction

* Use a --filter example without a space

It does not seem to work reliably and the docs do
not indicate it's supported

* Add how to format code

CONTRIBUTING.md

@@ -155,6 +155,9 @@ let ``preserve compile directive between piped functions, 512`` () = ...
 - Not mandatory, but when fixing a bug consider using `fix-<issue-number>` as the git branch name.<br />
 For example, `git checkout -b fix-1404`.
 
+- Code should be formatted to our standard style, using either `dotnet fake run build.fsx -t Format` which works on all files, or
+  `dotnet fake run build.fsx -t FormatChanged` to just change the files in git.
+
 ### Small steps
 
 It is better to create a draft pull request with some initial small changes, and engage conversation, than to spend a lot of effort on a large pull request that was never discussed.
@@ -243,12 +246,12 @@ In PowerShell:
 
 or in Bash:
 
-> VSTEST_HOST_DEBUG=1
+> export VSTEST_HOST_DEBUG=1
 
 Run a single unit test with `dotnet test --filter`.
 
 > cd .\src\Fantomas.Tests\
-> dotnet test --filter "record declaration"
+> dotnet test --filter 1700
 
 The output looks like: