Add additional docs on installations and releases. (#86)

Author: efritz

README.md

@@ -18,7 +18,13 @@ If there is something you'd like to see Sourcegraph be able to do from the CLI,
 
 ## Installation
 
-**NOTE:** To get the best version for _your_ Sourcegraph instance, simply replace `sourcegraph.com` in the commands below with your own Sourcegraph URL and the latest version compatible with your instance will be provided. If you are running a Sourcegraph version older than 3.13, run the commands below instead.
+For Sourcegraph 3.13 and newer, the preferred method of installation is to ask _your_ Sourcegraph instance for the latest compatible version. To do this, replace `https://sourcegraph.com` in the commands below with the address of your instance.
+
+For Sourcegraph 3.12 and older, run the following commands verbatim (against sourcegraph.com) or install from one of the published [releases on GitHub](https://github.com/sourcegraph/src-cli/releases).
+
+```
+https://github.com/sourcegraph/src-cli/releases/download/{version}/{binary}
+````
 
 #### Mac OS
 
@@ -108,4 +114,21 @@ go get -u github.com/sourcegraph/src-cli/cmd/src
 3.  Travis will automatically perform the release. Once it has finished, **confirm that the curl commands fetch the latest version above**.
 4.  Update the `MinimumVersion` constant in the [src-cli package](https://github.com/sourcegraph/sourcegraph/tree/master/internal/src-cli/consts.go).
 
-**Note**: The version recommended by a Sourcegraph instance will be the highest patch version with the same major and minor version as the set minimum. This means that patch versions are reserved solely for non-breaking changes and minor bug fixes. This allows us to dynamically release fixes for older versions of src-cli without having to update the instance.
+### Patch releases
+
+If a backwards-compatible change is made _after_ a backwards-incompatible one, the backwards-compatible one should be re-released to older instances that support it.
+
+A Sourcegraph instance returns the highest patch version with the same major and minor version as `MinimumVersion` as defined in the instance. Patch versions are reserved solely for non-breaking changes and minor bug fixes. This allows us to dynamically release fixes for older versions of `src-cli` without having to update the instance.
+
+To release a bug fix or a new feature that is backwards compatible with one of the previous two minor version of Sourcegraph, cherry-pick the changes into a patch branch and re-releases with a new patch version. 
+
+For example, suppose we have the the recommended versions.
+
+| Sourcegraph version | Recommended src-cli version |
+| ------------------- | --------------------------- | 
+| `3.100`             | `3.90.5`                    |
+| `3.99`              | `3.85.7`                    |
+
+If a new feature is added to a new `3.91.6` release of src-cli and this change requires only features available in Sourcegraph `3.99`, then this feature should also be present in a new `3.85.8` release of src-cli. Because a Sourcegraph instance will automatically select the highest patch version, all non-breaking changes should increment only the patch version. 
+
+Note that if instead the recommended src-cli version for Sourcegraph `3.99` was `3.90.4` in the example above, there is no additional step required, and the new patch version of src-cli will be available to both Sourcegraph versions.