Update README.md (#372)

## Why
Because when using a JSON env var it fails with a message that says:

```
google-github-actions/deploy-cloud-functions failed with: failed to parse KEY=VALUE pair ""my_key":"my_value"": missing "="
```

This was happening because an env var was a JSON string and commas were
not escaped. Therefore when parsing the env vars it failed when reaching
the second key of the JSON string.

## What
Adds a clarification to the "env_vars" parameter so that the user knows
they need to escape their commas. The clarification was borrowed from
the [source code of the function that parses the env
vars](https://github.com/google-github-actions/actions-utils/blob/main/src/kv.ts#L43)

Signed-off-by: Juan Manuel Ramallo <juanmanuelramallo@hey.com>

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Signed-off-by: Juan Manuel Ramallo <juanmanuelramallo@hey.com>

Author: juanmanuelramallo

README.md

@@ -61,7 +61,7 @@ jobs:
 
 - `region`: (Optional) [Region](https://cloud.google.com/functions/docs/locations) in which the function should be deployed. Defaults to `us-central1`.
 
-- `env_vars`: (Optional) List of key-value pairs to set as environment variables in the format: `KEY1=VALUE1,KEY2=VALUE2`. All existing environment variables will be removed, even if this parameter is not passed.
+- `env_vars`: (Optional) List of key-value pairs to set as environment variables in the format: `KEY1=VALUE1,KEY2=VALUE2`. All existing environment variables will be removed, even if this parameter is not passed. Keys or values that contain a separator must be escaped with a backslash (`\,`, `\\n`). All leading and trailing whitespace is trimmed.
 
 - `env_vars_file`: (Optional) Path to a local YAML file with definitions for all environment variables. An example env_vars_file can be found [here](tests/env-var-files/test.good.yaml). All existing environment variables will be removed, even if this parameter is not passed. If `env_vars` is also given, values in `env_vars` take precendence over these values.