Add known issues doc

docs/known-issues.md

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+# Known Issues
+
+## Output gets truncated
+
+This is a known issue with `console.log()` (see [nodejs/node#6379](https://github.com/nodejs/node/issues/6379)). 
+It's caused by different behaviour of `console.log()` writing to the terminal vs
+to a file. If a process calls `process.exit()`, buffered output will be truncated.
+To prevent this, the process should use `process.exitCode = 1` and wait for the 
+process to exit itself. Or use something like [exit](https://www.npmjs.com/package/exit) package.
+
+Workaround is to write to a temp file:
+```js
+let tmp = await $`mktemp` // Creates a temp file.
+let {stdout} = await $`cmd > ${tmp}; cat ${tmp}`
+```
+
+## Colors in subprocess
+
+You may see what tools invoked with `await $` don't show colors, compared to 
+what you see in a terminal. This is because, the subprocess does not think it's 
+a TTY and the subprocess turns off colors. Usually there is a way force 
+the subprocess to add colors.
+
+```js
+process.env.FORCE_COLOR='1'
+await $`cmd`
+```