DOC: Pandoc installation instructions missing from development environment setup guide
#### Problem description

The [development environment creation docs](http://pandas-docs.github.io/pandas-docs-travis/contributing.html#creating-a-development-environment) are currently missing instructions for installing [Pandoc](https://pandoc.org/), which is [required to convert Jupyter Notebooks to HTML when building the documentation](https://github.com/pandas-dev/pandas/blob/92372c7632a92d7bd980534b5a78590b2d9453ad/doc/make.py#L138-L151).

#### Expected Output

Pandoc should be added to `ci/requirements-optional-conda.txt` and a section should be added to the docs for folks using a `pip`-based environment to [follow the Pandoc installation instructions](https://pandoc.org/installing.html). This might apply to other non-Python dependencies like PostgreSQL for psycopg2 as well.

I'm happy to submit a PR for this.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: f9ea08c911f54d96c690312e0056859526eaff0b
python: 3.6.3.final.0
python-bits: 64
OS: Darwin
OS-release: 17.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0.dev0+133.gf9ea08c91
pytest: 3.2.4
pip: 9.0.1
setuptools: 36.7.2
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: 0.7.1
xarray: 0.9.6
IPython: 6.2.1
sphinx: 1.6.5
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.1.0
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.1.15
pymysql: 0.7.11.None
psycopg2: None
jinja2: 2.10
s3fs: 0.1.2
fastparquet: 0.1.3
pandas_gbq: None
pandas_datareader: None

</details>
