Add v1.0.0 CHANGELOG entry

Also change README formatting for Demo section

CHANGELOG.md

@@ -2,6 +2,22 @@
 
 ## [Unreleased][unreleased]
 
+(no unreleased changes)
+
+## [v1.0.0]
+
+**Release date: 2020-05-10**
+
+This 1.0 release is not a huge difference from the previous version,
+but it represents a commitment to support the API or announce any
+changes, as required by [semantic versioning](https://semver.org/).
+For this reason, some unused or unnecessary components have been
+removed from the public API, and several functions that are expected
+to be commonly used, such as `parse()` and `format()` for tree
+operations, have been given aliases in the top-level `penman`
+module. Consequently, there's less emphasis for creating a
+`PENMANCodec` object for most decoding or encoding tasks.
+
 ### Added
 
 * `penman.tree.Tree.walk()` ([#74])

README.md

@@ -87,13 +87,15 @@ $ echo "(w / want-01 :ARG0 (b / boy) :ARG1 (g / go :ARG0 b))" | penman --make-va
 ### Demo
 
 For a demonstration of the API usage, see the included
-[Jupyter](https://jupyter.org/) notebook at
-[docs/api-demo.ipynb](docs/api-demo.ipynb). You can view the notebook
-in GitHub by clicking the link, or you can run it interactively here
-(I suggest clearing the output before using it interactively: *Cell* >
-*All Output* > *Clear*):
+[Jupyter](https://jupyter.org/) notebook:
+
+- View it on GitHub: [docs/api-demo.ipynb](docs/api-demo.ipynb)
+- Run it on [mybinder.org](https://mybinder.org/):
+  [![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/goodmami/penman/master?filepath=docs%2Fapi-demo.ipynb)
+
+  (Note: clear the output before running: *Cell* > *All Output* >
+  *Clear*):
 
-[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/goodmami/penman/master?filepath=docs%2Fapi-demo.ipynb)
 
 ### PENMAN Notation