chore: start a style guide (#2683)

Co-authored-by: legendecas <[email protected]>

style.md

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+# OpenTelemetry JS Style Guide
+
+This guide is meant to be a supplement to the linting rules.
+It is not exhaustive, nor should the suggestions in this guide be considered hard rules.
+Suggestions for changes and additions to this doc are welcome and this doc should not be considered to be set in stone.
+There may be code written before this guide which does not follow the suggestions in this guide.
+That code is not required to be updated, but maybe a good starting place for new contributors getting used to the codebase.
+
+## Test coverage
+
+In general, all changes should be tested.
+New features generally require tests to be added and bugfixes require tests to ensure there are no regressions.
+
+## Linting
+
+The lint check must pass in order for a PR to be merged.
+In some cases, it may be acceptable to disable a linting rule for a specific line or file.
+It may also be acceptable in some cases to modify the linting rules themselves if a sufficient argument is made that the rule should be changed.
+
+## `null` and `undefined` should be treated the same
+
+In general, null and undefined should be treated the same.
+In case of the rare exception where `null` is used to mean something different than `undefined` it should be documented clearly in the jsdocs.
+
+- Prefer `undefined` instead of `null` in most cases
+- Prefer `value == null` instead of `value == null || value == undefined`
+
+## Prefer `===` over `==`
+
+`===`/`!==` should be preferred over `==` and `!=`.
+An exception to this is when checking for `null`/`undefined` when it is preferred to use `== null` in order to treat `null` and `undefined` the same.
+
+## Prefer options objects to long lists of optional positional parameters
+
+For functions/methods/constructors with optional parameters, the parameters should be passed as an options object.
+This allows options to be added later without changing the function signature and avoids long lists of arguments.
+Required arguments should be at the start of the arguments list.