gopls: document that v0.7.5 is the final version to support Go 1.12

For golang/go#50827

Change-Id: Ib59aa4431dcc492db9436c19f43872781679f789
Reviewed-on: https://go-review.googlesource.com/c/tools/+/382075
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/README.md

@@ -69,13 +69,21 @@ If you are having issues with `gopls`, please follow the steps described in the
 
 ## Supported Go versions and build systems
 
-`gopls` follows the [Go Release
-Policy](https://golang.org/doc/devel/release.html#policy), meaning that it
-officially supports the last 2 major Go releases. Per
+`gopls` follows the
+[Go Release Policy](https://golang.org/doc/devel/release.html#policy),
+meaning that it officially supports the last 2 major Go releases. Per
 [issue #39146](golang.org/issues/39146), we attempt to maintain best-effort
 support for the last 4 major Go releases, but this support extends only to not
 breaking the build and avoiding easily fixable regressions.
 
+The following table shows the final gopls version that supports being built at
+a given Go Version. Any more recent Go versions missing from this table can
+still be built with the latest version of gopls.
+
+| Go Version  | Final gopls Version With Support |
+| ----------- | -------------------------------- |
+| Go 1.12     | [gopls@v0.7.5](https://github.com/golang/tools/releases/tag/gopls%2Fv0.7.5) |
+
 Our extended support is enforced via [continuous integration with older Go
 versions](doc/contributing.md#ci). This legacy Go CI may not block releases:
 test failures may be skipped rather than fixed. Furthermore, if a regression in