Document Using Regex for str.split

```python
import pandas as pd
df = pd.DataFrame({'col': ['a-b-c+e=d,f#t']*5})
df.col.str.split('+|=', expand=True)

```
#### Problem description
While passing two patterns separating with `|`  to `str.split()` method, if one of them is `+`, `panads` returns the following error:
```
---------------------------------------------------------------------------
error                                     Traceback (most recent call last)
 in 
      2 import pandas as pd
      3 df = pd.DataFrame({'col': ['a-b-c+e=d,f#t']*5})
----> 4 df.col.str.split('+|=', expand=True)

~\Anaconda3\lib\site-packages\pandas\core\strings.py in split(self, pat, n, expand)
   2328     @copy(str_split)
   2329     def split(self, pat=None, n=-1, expand=False):
-> 2330         result = str_split(self._data, pat, n=n)
   2331         return self._wrap_result(result, expand=expand)
   2332 

~\Anaconda3\lib\site-packages\pandas\core\strings.py in str_split(arr, pat, n)
   1458             if n is None or n == -1:
   1459                 n = 0
-> 1460             regex = re.compile(pat)
   1461             f = lambda x: regex.split(x, maxsplit=n)
   1462     res = _na_map(f, arr)

~\Anaconda3\lib\re.py in compile(pattern, flags)
    231 def compile(pattern, flags=0):
    232     "Compile a regular expression pattern, returning a pattern object."
--> 233     return _compile(pattern, flags)
    234 
    235 def purge():

~\Anaconda3\lib\re.py in _compile(pattern, flags)
    299     if not sre_compile.isstring(pattern):
    300         raise TypeError("first argument must be string or compiled pattern")
--> 301     p = sre_compile.compile(pattern, flags)
    302     if not (flags & DEBUG):
    303         if len(_cache) >= _MAXCACHE:

~\Anaconda3\lib\sre_compile.py in compile(p, flags)
    560     if isstring(p):
    561         pattern = p
--> 562         p = sre_parse.parse(p, flags)
    563     else:
    564         pattern = None

~\Anaconda3\lib\sre_parse.py in parse(str, flags, pattern)
    853 
    854     try:
--> 855         p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, 0)
    856     except Verbose:
    857         # the VERBOSE flag was switched on inside the pattern.  to be

~\Anaconda3\lib\sre_parse.py in _parse_sub(source, state, verbose, nested)
    414     while True:
    415         itemsappend(_parse(source, state, verbose, nested + 1,
--> 416                            not nested and not items))
    417         if not sourcematch("|"):
    418             break

~\Anaconda3\lib\sre_parse.py in _parse(source, state, verbose, nested, first)
    614             if not item or (_len(item) == 1 and item[0][0] is AT):
    615                 raise source.error("nothing to repeat",
--> 616                                    source.tell() - here + len(this))
    617             if item[0][0] in _REPEATCODES:
    618                 raise source.error("multiple repeat",

error: nothing to repeat at position 0
```

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: 3.7.1
pip: 18.1
setuptools: 40.2.0
Cython: 0.29.2
numpy: 1.15.4
scipy: 1.2.0
pyarrow: None
xarray: 0.11.0
IPython: 7.1.1
sphinx: 1.7.6
patsy: 0.5.1
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.9
feather: None
matplotlib: 3.0.2
openpyxl: 2.5.5
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.5
lxml: 4.2.4
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: 1.2.10
pymysql: None
psycopg2: 2.7.6.1 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
