Merge develop to master with documentation changes.

docs/installation.md

+# Installation
+Clone this repository and install COMBO (we suggest using virtualenv/conda with Python 3.6+):
+```bash
+git clone https://github.com/ipipan/combo.git
+cd combo
+python setup.py develop
+combo --helpfull
+```
+
+## Problems & solutions
+* **jsonnet** installation error
+
+use `conda install -c conda-forge jsonnet=0.15.0`