Add a table for compatibility of PyGMT with Python and GMT (#763)

Author: seisman

README.rst

@@ -207,6 +207,7 @@ Other Python wrappers for GMT:
 
 Documentation for other versions
 --------------------------------
+
 * `Development <https://www.pygmt.org/dev>`__ (reflects the *master* branch on
   GitHub)
 * `Latest release <https://www.pygmt.org/latest>`__
@@ -216,3 +217,14 @@ Documentation for other versions
 * `v0.1.1 <https://www.pygmt.org/v0.1.1>`__
 * `v0.1.0 <https://www.pygmt.org/v0.1.0>`__
 * `v0.0.1a0 <https://www.pygmt.org/0.0.1a0>`__
+
+Compatibility with Python and GMT versions
+------------------------------------------
+
+======= ========== =========
+PyGMT   GMT        Python
+======= ========== =========
+0.2.1   >=6.1.1    >=3.6
+0.2.0   >=6.1.1    3.6 - 3.8
+0.1.x   >=6.0.0    3.6 - 3.8
+======= ========== =========