Specify Node version in README

[riordant]

Tried running a basic example and received

/home/riordant/Repositories/Personal/langchain/node_modules/langchain/dist/util/axios-fetch-adapter.cjs:239
    const headers = new Headers(config.headers);
                    ^
ReferenceError: Headers is not defined

As per https://github.com/hwchase17/langchainjs/issues/159, I was using and outdated Node version (16). So best to put min node version required in the docs/README.

[LarchLiu]

There should be a warning when running the project with Node v16, as the package.json file has defined a required node version of at least 18.

Unsupported engine: wanted: {"node":">=18"} (current: {"node":"v16.17.1","pnpm":"8.2.0"})

[dosubot[bot]]

Hi, @riordant! I'm here to help the LangChain team manage their backlog and I wanted to let you know that we are marking this issue as stale.

From what I understand, the issue you opened was about specifying the minimum required Node version in the README. LarchLiu commented that there should be a warning when running the project with Node v16, as the package.json file has defined a required node version of at least 18. This suggestion received a positive reaction from rozek.

Before we close this issue, we wanted to check with you if it is still relevant to the latest version of the LangChain repository. If it is, please let us know by commenting on the issue. Otherwise, feel free to close the issue yourself or it will be automatically closed in 7 days.

Thank you for your contribution to LangChain!