add conda-forge to install instructions

Following https://github.com/EpistasisLab/tpot/issues/165.
Opening this issue to dicuss conda-forge install instructions.

docs at http://epistasislab.github.io/tpot/installing/ and source at https://github.com/EpistasisLab/tpot/blob/master/docs_sources/installing.md

I just checked what packages are install with conda:

```
(tpot) ray@ray-MS-7B43:~$ conda install -c conda-forge tpot
Collecting package metadata (current_repodata.json): done
Solving environment: done

## Package Plan ##

environment location: /home/ray/local/bin/anaconda3/envs/tpot

added / updated specs:
- tpot

The following packages will be downloaded:

package | build
---------------------------|-----------------
certifi-2019.11.28 | py37_0 148 KB conda-forge
scikit-learn-0.22.1 | py37hcdab131_1 7.1 MB conda-forge
six-1.14.0 | py37_0 23 KB conda-forge
tpot-0.11.1 | py_0 75 KB conda-forge
tqdm-4.41.1 | py_0 45 KB conda-forge
------------------------------------------------------------
Total: 7.4 MB

The following NEW packages will be INSTALLED:

cffi conda-forge/linux-64::cffi-1.13.2-py37h8022711_0
chardet conda-forge/linux-64::chardet-3.0.4-py37_1003
cryptography conda-forge/linux-64::cryptography-2.8-py37h72c5cf5_1
deap conda-forge/linux-64::deap-1.3.0-py37hb3f55d8_0
idna conda-forge/linux-64::idna-2.8-py37_1000
joblib conda-forge/noarch::joblib-0.14.1-py_0
libblas conda-forge/linux-64::libblas-3.8.0-14_openblas
libcblas conda-forge/linux-64::libcblas-3.8.0-14_openblas
libgfortran-ng conda-forge/linux-64::libgfortran-ng-7.3.0-hdf63c60_4
liblapack conda-forge/linux-64::liblapack-3.8.0-14_openblas
libopenblas conda-forge/linux-64::libopenblas-0.3.7-h5ec1e0e_6
numpy conda-forge/linux-64::numpy-1.17.3-py37h95a1406_0
pandas conda-forge/linux-64::pandas-0.25.3-py37hb3f55d8_0
pycparser conda-forge/linux-64::pycparser-2.19-py37_1
pyopenssl conda-forge/linux-64::pyopenssl-19.1.0-py37_0
pysocks conda-forge/linux-64::pysocks-1.7.1-py37_0
python-dateutil conda-forge/noarch::python-dateutil-2.8.1-py_0
pytz conda-forge/noarch::pytz-2019.3-py_0
requests conda-forge/linux-64::requests-2.22.0-py37_1
scikit-learn conda-forge/linux-64::scikit-learn-0.22.1-py37hcdab131_1
scipy conda-forge/linux-64::scipy-1.4.1-py37h921218d_0
six conda-forge/linux-64::six-1.14.0-py37_0
stopit conda-forge/noarch::stopit-1.1.2-py_0
tpot conda-forge/noarch::tpot-0.11.1-py_0
tqdm conda-forge/noarch::tqdm-4.41.1-py_0
update_checker conda-forge/noarch::update_checker-0.16-py_0
urllib3 conda-forge/linux-64::urllib3-1.25.7-py37_0

The following packages will be UPDATED:

ca-certificates pkgs/main::ca-certificates-2019.11.27~ --> conda-forge::ca-certificates-2019.11.28-hecc5488_0

The following packages will be SUPERSEDED by a higher-priority channel:

certifi pkgs/main --> conda-forge
openssl pkgs/main::openssl-1.1.1d-h7b6447c_3 --> conda-forge::openssl-1.1.1d-h516909a_0

Proceed ([y]/n)?
```

The other optional packages not included are:
xgboost
dask
dask-ml
fsspec
scikit-mdr
skrebate

WIP: I'll check to see if the optional packages above are on conda-forge to provide other conda forge install examples e.g. `conda install -c conda-forge tpot dask`