fix typo, add docs

Author: njzjz

README.md

@@ -252,3 +252,14 @@ print(syst.get_charge()) # return the total charge of the system
 ```
 
 If a valence of 3 is detected on carbon, the formal charge will be assigned to -1. Because for most cases (in alkynyl anion, isonitrile, cyclopentadienyl anion), the formal charge on 3-valence carbon is -1, and this is also consisent with the 8-electron rule.
+
+# Plugins
+
+One can follow [a simple example](plugin_example/) to add their own format by creating and installing plugins. It's crirical to add the [Format](dpdata/format.py) class to `entry_points['dpdata.plugins']` in `setup.py`:
+```py
+    entry_points={
+        'dpdata.plugins': [
+            'random=dpdata_random:RandomFormat'
+        ]
+    },
+```

dpdata/plugins/gromacs.py

@@ -4,7 +4,7 @@
 
 @Format.register("gro")
 @Format.register("gromacs/gro")
-class PwmatOutputFormat(Format):
+class GromacsGroFormat(Format):
     def from_system(self, file_name, format_atom_name=True, **kwargs):
         """
         Load gromacs .gro file

dpdata/plugins/xyz.py

@@ -4,7 +4,7 @@
 
 @Format.register("quip/gap/xyz")
 @Format.register("quip/gap/xyz_file")
-class DeePMDRawFormat(Format):
+class QuipGapXYZFormat(Format):
     def from_labeled_system(self, data, **kwargs):
         return data
 

plugin_example/README.md

@@ -11,12 +11,14 @@ print(dpdata.System(12, fmt="random"))
 
 The output is
 
-> Data Summary
-> Unlabeled System
-> -------------------
-> Frame Numbers     : 12
-> Atom Numbers      : 20
-> Element List      :
-> -------------------
-> X
-> 20
+```
+Data Summary
+Unlabeled System
+-------------------
+Frame Numbers     : 12
+Atom Numbers      : 20
+Element List      :
+-------------------
+X
+20
+```