Quick conda install instructions in main README (#1717)

Co-authored-by: Dongdong Tian <[email protected]>

Author: weiji14

README.rst

@@ -77,6 +77,19 @@ Project goals
   ``xarray.DataArray`` for grids.
 
 
+Quickstart
+----------
+
+Installation
+++++++++++++
+
+Simple installation using `conda <https://docs.conda.io/projects/conda/en/latest/user-guide/index.html>`__::
+
+    conda install --channel conda-forge pygmt
+
+For other ways to install ``pygmt``, see `full installation instructions <https://www.pygmt.org/latest/install.html>`__.
+
+
 Contacting Us
 -------------