Homebrew: could not symlink, /usr/local/bin is not writable

While installing tig , HomeBrew is displaying the following issues while installing a dependency.

Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/a2x
/usr/local/bin is not writable.

You can try again using:
  brew link asciidoc

Following Alex's answer I was able to resolve this issue; seems this to be an issue non specific to the packages being installed but of the permissions of homebrew folders.

sudo chown -R `whoami`:admin /usr/local/bin

For some packages, you may also need to do this to /usr/local/share or /usr/local/opt .

sudo chown -R `whoami`:admin /usr/local/share
sudo chown -R `whoami`:admin /usr/local/opt