docs: explain how to handle read-only sandbox paths (#221)

Running build without `--sandbox_writable_path=/path/to/cache` might
result in `error: unable to create compilation: ReadOnlyFileSystem`
which has been reported in Bazel Slack workspace. I have also personally
experienced this in a small project of mine.

I thought adding this to the docs might help the next user reading it
should they face with the same issue.

Author: AlexTereshenkov

README.md

@@ -97,6 +97,10 @@ build --repo_env=HERMETIC_CC_TOOLCHAIN_CACHE_PREFIX=/path/to/cache
 build --sandbox_add_mount_pair=/path/to/cache
 ```
 
+If you get an error `unable to create compilation: ReadOnlyFileSystem`, 
+try adding `build --sandbox_writable_path=/path/to/cache` to `.bazelrc` 
+which is known to resolve the issue for some users.
+
 The snippets above will download the zig toolchain and make the bazel
 toolchains available for registration and usage. If nothing else is done, this
 will work for some minimal use cases. The `.bazelrc` snippet instructs Bazel to